Patent Publication Number: US-2023144991-A1

Title: Chiller suction flow limiting with input power or motor current control

Description:
BACKGROUND 
     Refrigerants can transfer heat between fluids and may be employed in a variety of applications, such as heating, ventilating, and air conditioning (HVAC) systems, heat pumps, or power generation in Organic Rankine Cycles (ORE). The refrigerant can be transported within a refrigerant piping system, which includes pipes, pipe fittings, valves, and the like. The refrigerant piping system transports the refrigerant between various vessels and equipment within the HVAC system, such as compressors, turbines, pumps, evaporators, and condensers. The refrigerant may undergo one or more phase changes within the refrigerant piping system, such that liquid refrigerant and vaporous refrigerant may both be present in the HVAC system. 
     SUMMARY 
     One implementation of the present disclosure is a chiller. The chiller includes an evaporator that receives a first flow of refrigerant, transfers heat to the first flow of refrigerant, and outputs a second flow of refrigerant. The chiller includes a compressor that receives the second flow of refrigerant via tubing between the evaporator and the compressor, the compressor including a prime mover that performs work on the second flow of refrigerant based on at least one of an input power to the prime mover and an input current to the prime mover. The chiller includes a first pressure sensor that detects a first pressure of refrigerant in the evaporator. The chiller includes a second pressure sensor that detects a second pressure of refrigerant in a condenser of the chiller. The chiller includes a controller that determines a predicted energy level of operation of the compressor based on the first pressure and the second pressure, the predicted energy level associated with liquid droplet flow in the second flow of refrigerant received by the compressor, compares the predicted energy level to an operating energy level of the compressor, and modifies the at least one of the input power and the input current to the prime mover based on the comparison satisfying a modification condition. 
     Another implementation of the present disclosure is a method of chiller suction flow limiting. The method includes receiving, by a controller, a first pressure from an evaporator pressure sensor coupled to an evaporator. The method includes receiving, by the controller, a second pressure from a condenser pressure sensor coupled to a condenser. The method includes determining, by the controller, a predicted energy level of operation of a compressor based on the first pressure and the second pressure, the predicted energy level associated with liquid droplet flow from the evaporator to the condenser. The method includes comparing, by the controller, the predicted energy level to an operating energy level of the compressor. The method includes modifying at least one of an input power and an input current to a prime mover of the compressor based on the comparison satisfying a modification condition. 
     Still another implementation of the present disclosure is a chiller controller. The chiller controller includes one or more processors and a memory device storing computer-readable instructions that when executed by the one or more processors, cause the one or more processors to receive, at a state detector, a first pressure from an evaporator pressure sensor coupled to an evaporator; receive, at the state detector, a second pressure from a condenser pressure sensor coupled to a condenser; determine, by an energy predictor, a predicted energy level of operation of a compressor based on the first pressure and the second pressure, the predicted energy level associated with liquid droplet flow from the evaporator to the condenser; compare, by a compressor controller, the predicted energy level to an operating energy level; and modify; by the compressor controller, at least one of an input power and an input current to a prime mover of the compressor based on the comparison satisfying a modification condition. 
     Those skilled in the art will appreciate that the summary is illustrative only and is not intended to be in any way limiting. Other aspects, inventive features, and advantages of the devices and/or processes described herein, as defined solely by the claims, will become apparent in the detailed description set forth herein and taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a perspective view of a building serviced by a heating, ventilation, and air conditioning (HVAC) system, according to an exemplary embodiment. 
         FIG.  2    is a block diagram illustrating a portion of the HVAC system of  FIG.  1    in greater detail, showing a refrigeration circuit configured to circulate a refrigerant between an evaporator and a condenser, according to an exemplary embodiment. 
         FIG.  3    is a block diagram of a controller of the refrigeration circuit of  FIG.  2   , according to an exemplary embodiment. 
         FIG.  4    is a flow diagram of a method of chiller suction flow limiting with input power or motor current control, according to an exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure relates generally to the field of refrigeration systems. More particularly, the present disclosure relates to chiller suction flow limiting with input power or motor current control. A refrigeration system can include a chiller, which can include an evaporator, condenser, compressor, and tubing connecting these and various other components. The evaporator evaporates refrigerant to provide net cooling of process fluid, such as water, flowing through the tubing. It can be desirable for the evaporator to generate a dry, saturated vapor from the refrigerant, and for the compressor to thus receive the dry, saturated vapor based on suction generated by the compressor. However, in some situations, the refrigerant outputted by the evaporator includes liquid droplets, which are pulled up with the high velocity vapor flow based on the suction from the compressor. In addition, size, weight, power, and cost considerations may make it desirable to reduce the size of the evaporator to meet the minimum needs of a design capacity of the refrigeration system. However, as lift or differential pressure across the compressor is lowered due to chiller operating conditions, the compressor may provide a higher capacity and suction flow rate, which can increase gas velocity in the evaporator and cause liquid droplets to carry over into the compressor. These effects can decrease the efficiency of the chiller, and can damage mechanical components of the compressor. 
     The present solution can address such considerations by implementing chiller suction flow limiting with input power or motor current control, in order to effectively manage compressor operation to reduce or eliminate liquid droplet flow from the evaporator to the compressor. For example, systems and methods in accordance with the present solution can predict power or current levels corresponding to a design velocity limit of the evaporator, at which liquid droplet flow to the compressor could be expected to occur, and use a controller to limit further power or current increase to prevent the liquid droplet flow to the compressor (e.g., liquid flow carryover). In some embodiments, a chiller includes an evaporator that receives a first flow of refrigerant, transfers heat to the first flow of refrigerant, and outputs a second flow of refrigerant. The chiller includes a compressor that receives the second flow of refrigerant via tubing between the evaporator and the compressor, the compressor including a prime mover that performs work on the second flow of refrigerant based on at least one of an input power to the prime mover and an input current to the prime mover. The chiller includes a first pressure sensor that detects a first pressure of refrigerant in the evaporator. The chiller includes a second pressure sensor that detects a second pressure of refrigerant in a condenser of the chiller. The chiller includes a controller that determines a predicted energy level of operation of the compressor based on the first pressure and the second pressure, compares the predicted energy level to an operating energy level associated with liquid droplet flow in the second flow of refrigerant received by the compressor, and modifies the at least one of the input power and the input current to the prime mover based on the comparison satisfying a modification condition. As such, if the predicted energy level is too high (e.g., is greater than the operating energy level), the controller can limit the power or current, as appropriate, to the prime mover to reduce or eliminate a risk of liquid droplet flow into the compressor, which might otherwise occur if a design velocity limit of the evaporator is exceeded. 
     HVAC System 
       FIG.  1    depicts a perspective view of a building  10 . Building  10  is serviced by a heating, ventilation, and air conditioning system (HVAC) system  20 . HVAC system  20  can include a chiller  22 , a boiler  24 , a rooftop cooling unit  26 , and a plurality of air-handling units (AHUs)  36 . HVAC system  20  uses a fluid circulation system to provide heating and/or cooling for building  10 . The circulated fluid may be cooled in chiller  22  or heated in boiler  24 , depending on whether cooling or heating is required. Boiler  24  may add heat to the circulated fluid by burning a combustible material (e.g., natural gas). Chiller  22  may place the circulated fluid in a heat exchange relationship with another fluid (e.g., a refrigerant) in a heat exchanger (e.g., an evaporator). The refrigerant removes heat from the circulated fluid during an evaporation process, thereby cooling the circulated fluid. 
     The circulated fluid from chiller  22  or boiler  24  may be transported to AHUs  36  via piping  32 . AHUs  36  may place the circulated fluid in a heat exchange relationship with an airflow passing through AHUs  36 . For example, the airflow may be passed over piping in fan coil units or other air conditioning terminal units through which the circulated fluid flows. AHUs  36  may transfer heat between the airflow and the circulated fluid to provide heating or cooling for the airflow. The heated or cooled air may be delivered to building  10  via an air distribution system including air supply ducts  38  and may return to AHUs  36  via air return ducts  40 . HVAC system  20  can include a separate  36  on each floor of building  10 . In other embodiments, a single AHU (e.g., a rooftop AHU) may supply air for multiple floors or zones. The circulated fluid from AHUs  36  may return chiller  22  or boiler  24  via piping  34 . 
     [NU] The refrigerant in chiller  22  can be vaporized upon absorbing heat from the circulated fluid. The vapor refrigerant may be provided to a compressor within chiller  22  where the temperature and pressure of the refrigerant are increased (e.g., using a rotating impeller, a screw compressor, a scroll compressor, a reciprocating compressor, a centrifugal compressor, etc.). The compressed refrigerant may be discharged into a condenser within chiller  22 . In some embodiments, water (or another fluid) flows through tubes in the condenser of chiller  22  to absorb heat from the refrigerant vapor, thereby causing the refrigerant to condense. The water flowing through tubes in the condenser may be pumped from chiller  22  to a cooling unit  26  via piping  28 . Cooling unit  26  may use fan driven cooling or fan driven evaporation to remove heat from the water. The cooled water from cooling unit  26  may be delivered back to chiller  22  via piping  30  and the cycle repeats. 
       FIG.  2    depicts a block diagram illustrating a portion of HVAC system  20 , according to an exemplary embodiment. Chiller  22  can include a refrigeration circuit  42  and a controller  100 . Refrigeration circuit  42  can include an evaporator  46 , a compressor  48 , a condenser  50 , and an expansion valve  52 . Compressor  48  may be configured to circulate a refrigerant through refrigeration circuit  42 . Compressor  48  can be operated by controller  100 . Compressor  48  may compress the refrigerant to a high pressure, high temperature state and discharge the compressed refrigerant into a compressor discharge line  54  connecting the outlet of compressor  48  to the inlet of condenser  50 . The compressor  48  can be or include a screw compressor, a semi-hermetic screw compressor, or compressor  48  is a hermitic or open screw compressor, for example. Compressor  48  can also be or include a scroll compressor, a reciprocating compressor, a centrifugal compressor, or still another type of compressor. 
     Condenser  50  may receive the compressed refrigerant from compressor discharge line  54 . Condenser  50  may also receive a separate heat exchange fluid from cooling circuit  56  (e.g., water, a water-glycol mixture, another refrigerant, etc.). Condenser  50  may, be configured to transfer heat from the compressed refrigerant to the heat exchange fluid, thereby causing the compressed refrigerant to condense from a gaseous refrigerant to a liquid or mixed fluid state. The cooling circuit  56  can include a heat recovery circuit configured to use the heat absorbed from the refrigerant for heating applications. The cooling circuit  56  can include a pump  58  for circulating the heat exchange fluid between condenser  50  and cooling unit  26 . Cooling unit  26  may include cooling coils  60  configured to facilitate heat transfer between the heat exchange fluid and another fluid (e.g., air) flowing through cooling unit  26 . The cooling unit  26  can include a cooling tower. The heat exchange fluid can reject heat in cooling unit  26  and return to condenser  50  via piping  30 . 
     The refrigeration circuit  42  can include a line  62  connecting an outlet of condenser  50  to an inlet of expansion device  52 . Expansion device  52  can expand the refrigerant in refrigeration circuit  42  to a low temperature and low pressure state. Expansion device  52  may be a fixed position device or variable position device (e.g., a valve). Expansion device  52  may be actuated manually or automatically (e.g., by controller  100  via a valve actuator) to adjust the expansion of the refrigerant passing therethrough. Expansion device  52  may output the expanded refrigerant into line  64  connecting an outlet of expansion device  52  to an inlet of evaporator  46 . 
     Evaporator  46  may receive the expanded refrigerant from line  64 . Evaporator  46  may also receive a separate chilled fluid from chilled fluid circuit  66  (e.g., water, a water-glycol mixture, another refrigerant, etc.). Evaporator  46  may be configured to transfer heat from the chilled fluid to the expanded refrigerant in refrigeration circuit  42 , thereby cooling the chilled fluid and causing the refrigerant to evaporate. The chilled fluid circuit  66  can include a pump  68  for circulating the chilled fluid between evaporator  46  and AHU  36 . AHU  36  may include cooling coils  70  configured to facilitate heat transfer between the chilled fluid and another fluid (e.g., air) flowing through AHU  36 . The chilled fluid may absorb heat in AEU  36  and return to evaporator  46  via piping  34 . Evaporator  46  may output the heated refrigerant to compressor suction line  72  connecting the outlet of evaporator  46  with the inlet of compressor  48 . 
     The chilled fluid circuit  66  can include a chilled fluid temperature sensor  74  positioned along piping  32 . Chilled fluid temperature sensor  74  may be configured to detect a temperature T cf  of the chilled fluid (e.g., the leaving chilled liquid temperature, etc.) flowing within piping  32  between evaporator  46  and TT  36 . The refrigeration circuit  42  can include a suction temperature sensor  76  positioned along compressor suction line  72 . Suction temperature sensor  76  may be configured to detect a temperature T suc  of the refrigerant flowing within compressor suction line  72  between evaporator  46  and compressor  48  (i.e., the temperature of the refrigerant entering compressor  48 ). The refrigeration circuit  42  can include a suction pressure sensor  78  positioned along compressor suction line  72 . Suction pressure sensor  78  may be configured to detect a pressure P suc  of the refrigerant flowing within compressor suction line  72  between evaporator  46  and compressor  48  (i.e., the pressure of the refrigerant entering compressor  48 ). The refrigeration circuit  42  can include a discharge temperature sensor  80  positioned along compressor discharge line  54 . Discharge temperature sensor  80  may be configured to detect a temperature T dis  of the refrigerant flowing within compressor discharge line  54  between compressor  48  and condenser  50  (i.e., the temperature of the refrigerant exiting compressor  48 ). The refrigeration circuit  42  can include a discharge pressure sensor  82  positioned along compressor discharge line  54 . Discharge pressure sensor  82  may be configured to detect a pressure P dis  of the refrigerant flowing within compressor discharge line  54  between compressor  48  and condenser  50  (i.e., the pressure of the refrigerant exiting compressor  48 ). 
     Refrigeration circuit  42  can include an evaporator pressure sensor  86  that detects a pressure P evap  of the refrigerant flowing within evaporator  46 , and a condenser pressure sensor  88  that detects a pressure P cond  of the refrigerant flowing within condenser  50 . Sensors  86 ,  88  may be similar to sensors  78 ,  82 ; sensors  78 ,  82  may respectively be used to perform the functions of sensors  86 ,  88  relating to measuring pressures associated with evaporator  46  and compressor  48  as described further herein. Sensors  86 ,  88  may be positioned at various points in or adjacent to evaporator  46  and condenser  50 , respectively, to detect respective pressure P evap  and P cond . 
     Compressor  48  includes a prime mover  84  (e.g., a motor). The prime mover  84  can be a fixed speed drive or a variable speed drive. Controller  100  can control operation of prime mover  84 , such as to transmit control signals to prime mover  84  to control a rotation speed, flow rate, or other operational parameter of compressor  48 . The controller  100  can control operation of prime mover  84  based on at least one of a power or a current corresponding to operation of compressor  48 , Depending on operational conditions in refrigeration circuit  42 , liquid droplets of refrigerant may flow from evaporator  46  to compressor  48 . Controller  100  can control operation of prime mover  84  to reduce or eliminate liquid droplet flow from evaporator  46  to compressor  48 . 
     Chiller Suction Flow Limiting with Input Power or Motor Current Control 
       FIG.  3    depicts a block diagram of a refrigeration system  150  including controller  100 , according to an exemplary embodiment. Controller  100  can control operation of compressor  48 , such as to control operation of compressor  48  based on at least one of input power or motor current control. Power and current, such as input power, input current, motor power, motor current, can include power or current to the refrigeration system  150  (e.g., to the chiller), power or current to a motor controller of the refrigeration system  150 , power or current to a drive of compressor  48  (e.g., variable speed drive), power or current to a motor of compressor  48 , or other power or current used to cause compressor  48  to move. Controller  100  can use power and current limits to protect compressor  48  (e.g., prime mover  84 ), or limit building energy usage, and such limits can be very stable in terms of control methodology (e.g., controller  100  need not rely on detected liquid droplets as an input to a feedback control loop, and thus can prevent liquid droplet carryover before it occurs). For example, controller  100  can control a power of operation of a variable speed drive prime mover  84 , or can control a current of operation of a fixed speed drive prime mover  84 . Controller  100  can determine a predicted energy level of operation of compressor  48  at which suction flow from evaporator  46  to compressor  48  might be expected to cause liquid droplets to flow into compressor  48 , compare the predicted energy level to an actual operating energy level of compressor  48 , and determine to limit capacity of compressor  48  based on the comparison to protect compressor  48  from liquid droplet flow. 
     The controller  100  can include a communications interface  102  and a processing circuit  104 . Communications interface  102  may include wired or wireless interfaces (e.g., jacks, antennas, transmitters, receivers, transceivers, wire terminals, etc.) for conducting data communications with various systems, devices, or networks. For example, communications interface  102  may include an Ethernet card and/or port for sending and receiving data via an Ethernet-based communications network. In some embodiments, communications interface  102  includes a wireless transceiver (e.g., a WiFi transceiver, a Bluetooth transceiver, a NEC transceiver, ZigBee, etc.) for communicating via a wireless communications network. Communications interface  102  may be configured to communicate via local area networks (e.g., a building LAN, etc.) and/or wide area networks (e.g., the Internet, a cellular network, a radio communication network, etc.) and may use a variety of communications protocols (e.g., BACnet, TCP/IP, point-to-point, etc.). 
     The communications interface  102  can facilitate receiving inputs from various sensors. The sensors may include, for example, chilled fluid temperature sensor  74  configured to detect the temperature of the chilled fluid at an outlet of evaporator  46 , suction pressure sensor  78  configured to detect the pressure of the refrigerant in compressor suction line  72 , discharge pressure sensor  82  configured to detect the pressure of the refrigerant in compressor discharge line  54 , and/or other sensors of chiller  22  and/or HVAC system  20  (e.g., suction temperature sensor  76 , discharge temperature sensor  80 , chilled fluid temperature sensor  74 , etc.). Communications interface  102  may receive the inputs directly from the sensors, via a local network, and/or via a remote communications network. Communications interface  102  may enable communications between controller  100  and compressor  48 . 
     The processing circuit  104  can include a processor  106  and memory  108 . Processor  106  may be a general purpose or specific purpose processor, an application specific integrated circuit (ASIC), one or more field programmable gate arrays (FPGAs), a group of processing components, or other suitable processing components. Processor  106  may be configured to execute computer code or instructions stored in memory  108  (e.g., fuzzy logic, etc.) or received from other computer readable media (e.g., CDROM, network storage, a remote server, etc.) to perform one or more of the processes described herein. 
     Memory  108  may include one or more data storage devices (e.g., memory units, memory devices, computer-readable storage media, etc.) configured to store data, computer code, executable instructions, or other forms of computer-readable information. Memory  108  may include random access memory (RAM), read-only memory (ROM), hard drive storage, temporary storage, non-volatile memory, flash memory, optical memory, or any other suitable memory for storing software objects and/or computer instructions. Memory  108  may include database components, object code components, script components, or any other type of information structure for supporting the various activities and information structures described in the present disclosure. Memory  108  may be communicably connected to processor  106  via processing circuit  104  and may include computer code for executing (e.g., by processor  106 ) one or more of the processes described herein. 
     The memory  108  can includes various modules for completing processes described herein. More particularly, memory  108  includes a state detector  110 , an energy predictor  112 , and a compressor controller  114 . While various modules with particular functionality are shown in  FIG.  3   , controller  100  and memory  108  may include any number of modules for completing the functions described herein. For example, the activities of multiple modules may be combined as a single module and additional modules with additional functionality may be included. The controller  100  can further control other processes beyond the scope of the present disclosure, including but not limited to controlling operation of various components of refrigeration system  150  based on a desired or expected load condition. 
     State detector  110  can receive state data from various sensors of refrigeration system  150 . For example, state detector  110  can receive pressure data from evaporator pressure sensor  86  and from condenser pressure sensor  88 . State detector  110  can also receive temperature data from temperature sensors. 
     Energy predictor  112  can receive the state data from state detector  110  and determine a predicted energy level of operation of compressor  48  based on the received state data. The predicted energy level can correspond to at least one of compressor speed, compressor capacity, water flow rate, water temperature, suction volume flow rate, compressor performance, motor performance, and starter performance. 
     Energy predictor  112  can determine the predicted energy level based on a first pressure received by state detector  110  from evaporator pressure sensor  86  and a second pressure received by state detector  110  from condenser pressure sensor  88 . Energy predictor  112  can execute an energy prediction function to calculate the predicted energy level. 
     The energy prediction function may include one or more calculation parameters that the energy predictor  112  can apply to the first pressure and second pressure to calculate the predicted energy level. The one or more calculation parameters can be determined based on experimental and/or simulation testing of operation of refrigeration system  150 . For example, the one or more calculation parameters can be determined by identifying energy levels associated with various values of evaporator pressure and condenser pressure, and fitting a curve, function, or other representation to the energy levels based on the values of evaporator pressure and condenser pressure. The energy levels may be identified by operating the refrigeration system  150  (or a driveline thereof) at various operating conditions. The energy levels may be identified by operating the refrigeration system  150  at part-load conditions, which may provide a more accurate representation of the behavior of the refrigeration system when the one or more calculation parameters are used to predict the energy levels. It will be appreciated that calculation parameter(s) determined for a first refrigeration system  150  may be applied to various other refrigeration systems  150 . The one or more calculation parameters may be determined for a specific refrigeration system using inputs such as capacity, water flow rates, and water temperatures, with feedback values such as evaporator pressure, condenser pressure, suction volume flow rate, and input current (or input power), and executing an iterative process due to dependencies between evaporator pressure (or saturation temperature), suction volume flow limit, capacity, and desired volume flow rate. Where the calculation parameters are determined based on a driveline of the refrigeration system  150  (e.g., to extrapolate the determined calculation parameters to other units having a similar driveline), performance parameters (e.g., compressor performance, motor performance, starter performance) can be determined based on boundary condition variables of compressor  48  (e.g., suction pressure, volume flow rate (or non-dimensional flow rate, theta), and discharge pressure (or non-dimensional head, omega)), to determine the corresponding input current (or input power). As such, if the values of the boundary condition variables (e.g., volume flow rate or theta are) selected to be at the appropriate limit values, then the driveline calculations can directly provide the data needed to determine the calculation parameters. It will be appreciated that the calculation parameters can be determined using processing circuit  104 , or a processing circuit of a device remote from refrigeration system  150  (or from a driveline) that is operated to identify relationships between evaporator pressure, condenser pressure, and liquid droplet flow. 
     Energy predictor  112  can select a particular energy prediction function to execute based on an operating characteristic of compressor  48 , which may be stored by energy predictor  112 . The operating characteristic may indicate whether prime mover  84  of compressor  48  operates in a variable speed mode of operation or a fixed speed mode of operation. If the operating characteristic indicates that prime mover  84  operates in a variable speed mode of operation, energy predictor  112  can select the energy prediction function according to Equation 1: 
     
       
         
           
             
               
                 
                   
                     E 
                     VSD 
                   
                   = 
                   
                     
                       ( 
                       
                         a 
                         + 
                         
                           b 
                           · 
                           
                             p 
                             evap 
                           
                         
                       
                       ) 
                     
                     · 
                     
                       
                         ( 
                         
                           
                             p 
                             cond 
                           
                           
                             p 
                             evap 
                           
                         
                         ) 
                       
                       c 
                     
                   
                 
               
               
                 
                   Eqn 
                   . 
                       
                   1 
                 
               
             
           
         
       
     
     If the operating characteristic indicates that prime mover  84  operates in a fixed speed mode of operation, energy predictor  112  can select the energy prediction function according to Equation 2: 
     
       
         
           
             
               
                 
                   
                     E 
                     FSD 
                   
                   = 
                   
                     
                       ( 
                       
                         a 
                         + 
                         
                           b 
                           · 
                           
                             p 
                             evap 
                           
                         
                       
                       ) 
                     
                     · 
                     
                       ln 
                       ⁡ 
                       ( 
                       
                         c 
                         + 
                         
                           
                             p 
                             cond 
                           
                           
                             p 
                             evap 
                           
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   Eqn 
                   . 
                       
                   2 
                 
               
             
           
         
       
     
     As such, energy predictor  112  can execute the appropriate energy prediction function using the calculation parameters and the first and second pressures (e.g., p evap =first pressure, p cond =second pressure) to calculate the predicted energy level. It will be appreciated that the values of the calculation parameters can be determined by fitting curves of the form shown in Equation 1 or Equation 2, as appropriate, to identified values of energy level as a function of evaporator pressure and condenser pressure. An iterative optimization process may be used to determine the values of the calculation parameters. The functions shown in Equation 1 and Equation 2 may be linearized (e.g., by taking a logarithm, such as the natural logarithm, of both sides of the respective equations) to reduce the computational requirements for determining the calculation parameters by enabling the use of a linear fit method, such as linear least squares methods. 
     Compressor controller  114  can control operation of compressor  48  (e.g., control operation of prime mover  84 ). Compressor controller  114  can output a control signal corresponding to a desired input power or input current to compressor  48 , including to limit the input power or input current as appropriate. Compressor controller  114  can use the operating characteristic of compressor  48  to determine whether to generate the control signal to control the input power (e.g., if compressor  48  operates in a variable speed mode of operation) or the input current (e.g., if compressor  48  operates in a fixed speed mode of operation). The compressor controller  114  may initially calculate the input power or the input current based on input variables such as desired water flow rates, water temperatures, or other variables representative of performance of the refrigeration system  150 . The compressor controller  114  can limit the initially calculated input power or input current to reduce or eliminate liquid droplet flow into compressor  48 . 
     Compressor controller  114  compares the predicted energy level determined by energy predictor  112  to an operating energy level. The predicted energy level may correspond to an energy level, given certain values of evaporator pressure and condenser pressure, at which liquid droplet flow into compressor  48  from evaporator  46  may be expected to occur. For example, the predicted energy level may correspond to an energy level at which a design velocity limit of evaporator  46  is exceeded, or at which liquid droplet flow has been determined to occur through experimental and/or simulation testing. The operating energy level can be a current energy level of compressor  48 ; as such, compressor controller  114  can use the comparison to determine whether compressor  48  is operating at a condition which may exceed the predicted energy level at which liquid droplet flow into compressor  48  from evaporator  46  may be expected to occur. 
     Compressor controller  114  can measure at least one of actual input current and actual input power. For example, compressor controller  114  can include an input current sensor, such as a current transformer, to measure actual input current. Compressor controller  114  can include an input power sensor, such as a voltage sensor that can be used to determined actual input power (e.g., based on the actual input current and the actual input power). Compressor controller  114  can determine the operating energy level based on the at least one of the actual input current and the actual input power. 
     Compressor controller  114  modifies at least one of the input power or the input current to compressor  48  based on the comparison satisfying a modification condition. For example, if the predicted energy level is a value that should not be exceeded, compressor controller  114  can limit the at least one of the input power or the input current responsive to the operating energy level exceeding the predicted energy level (e.g., if operating energy level is greater than predicted energy level, limit the at least one of the input power and the input current). If the predicted energy level is set to a value that triggers limiting, compressor controller  114  can limit the at least one of the input power and the input current responsive to the operating energy level equaling the predicted energy level (e.g., if predicted energy level is equal to operating energy level, limit the at least one of the input power and the input current). Compressor controller  114  can calculate the predicted energy level as at least one of a predicted input current and a predicted input power, such that compressor controller  114  can perform the comparison by comparing at least one of actual input current to predicted input current and actual input power to predicted input power. 
     If the comparison does not satisfy the modification condition; such as if the operating energy level is less than the predicted energy level, then compressor controller  114  can determine to not limit the input power or the input current; for example, compressor controller  114  can continue to monitor the first and second pressures; compressor controller  114  can determine to increase the at least one of the input power and the input current (if desired performance, such as water flow rates or water temperatures, is indicative of instructions to increase the at least one of the input power and the input current). As such, where measured values of input power and/or input current are above the predicted values, compressor controller  114  can reduce the operating capacity of compressor  48 ; where measured values of input power and/or input current are below the predicted values, the operating capacity of compressor  48  (and thus refrigeration system  150 ) is not limited by suction flow, and compressor controller  114  can control the at least one of the input power and the input current by executing various processes, such as by controlling the at least one of the input power and the input current based on T cf  of the chilled fluid leaving evaporator  46  as detected by chilled fluid temperature sensor  74  (e.g., by comparing Too a desired value of T cf ). Compressor controller  114  can execute capacity control of compressor  48  based on one or more of variation of compressor speed using a variable speed drive; compressor suction flow dampers or pre-rotation vane flow throttling; compressor discharge variable geometry diffuser flow throttling; or a capacity control slide valve (e.g., if compressor  48  includes a screw compressor). 
     Compressor controller  114  can limit the at least one of the input power or the input current by setting the at least one of the input power or the input current to a previous value. For example, compressor controller  114  can maintain a database of power and current values. Responsive to determining to limit the input power or the input current, compressor controller  114  can retrieve a previous value of input power or input current from the history, such as a previous value at a point in time at which compressor controller  114  determined not to modify the at least one of the input power or the input current based on a corresponding previous predicted energy level. 
     Compressor controller  114  can maintain a database including evaporator pressure, condenser pressure, input power, input current, predicted energy level, and various other operational parameters, along with an indication that the comparison indicated that the input power or input current was to be limited. 
     Compressor controller  114  can output an alert indicating liquid droplet flow may be occurring based on the comparison. For example, compressor controller  114  can cause communications interface  102  to transmit the alert. The alert may include information such as the operational parameters maintained in the database by compressor controller  114 . The alert may include an indication of a value of a performance variable corresponding to the modification condition being satisfied, such as a water flow rate or water temperature resulting in a predicted energy level associated with liquid droplet flow. 
       FIG.  4    depicts a method  400  of operating a refrigeration system (e.g., a chiller), according to an exemplary embodiment. The method  400  can be performed using the HVAC system of  FIG.  1    and/or the refrigeration system  150  of  FIGS.  2 - 3   . 
     At  405 , a first pressure is received by a controller from an evaporator pressure sensor. The first pressure can be representative of a pressure of refrigerant flowing through an evaporator. The evaporator can receive a first flow of refrigerant, transfer heat to the first flow of refrigerant, and output a second flow of refrigerant. 
     At  410 , a second pressure is received by the controller from a condenser pressure sensor. The second pressure can be representative of a pressure of refrigerant flowing through the condenser. 
     At  415 , the controller determines a predicted energy level of operation of a compressor. The compressor can receive the second flow of refrigerant via tubing between the evaporator and the compressor. The predicted energy level can be determined using an energy prediction function that uses the first pressure and the second pressure as inputs and evaluates the inputs using predetermined parameters. The compressor can include a prime mover that performs work on the second flow of refrigerant based on at least one of an input power to the prime mover and an input current to the prime mover. The prime mover can include a variable speed drive that the controller drives using input power. The prime mover can include a fixed speed drive that the controller drives using input current. 
     At  420 , the controller compares the predicted energy level of operation of the compressor to an operating energy level. The predicted energy level can be associated with liquid droplet flow in the second flow of refrigerant received by the compressor. The predicted energy level can correspond to a design velocity limit of the evaporator. The compressor controller can determine the operating energy level using at least one of an actual input current and an actual input power. 
     At  425 , the controller determines whether the comparison satisfies a modification condition. The modification condition can correspond to the operating energy level being greater than the predicted energy level, or the operating energy level being greater than or equal to the predicted energy level. 
     At  430 , the controller limits at least one of the input power and the input current to the prime mover based on the comparison satisfying a modification condition. The controller can modify the at least one of the input power and the input current to limit the at least one of the input power and the input current to a value at which liquid droplet flow from the evaporator into the compressor can be reduced or eliminated. The controller can output an alert responsive to modifying the at least one of the input power and the input current to the prime mover based on the comparison satisfying the modification condition. If the comparison does not satisfy the modification condition, such as if the operating energy level is less than the predicted energy level, then the controller can continue to monitor the pressures received from the evaporator pressure sensor and the condenser pressure sensor. If the comparison does not satisfy the modification condition, such as if the operating energy level is less than the predicted energy level, the controller can continue to increase the input power or input current, as appropriate, if the desired performance of the chiller (e.g., desired water flow rates or water temperatures) indicate instructions to increase the input power or input current. 
     References to “or” may be construed as inclusive so that any terms described using “or” may indicate any of a single, more than one, and all of the described terms. References to at least one of a conjunctive list of terms may be construed as an inclusive OR to indicate any of a single, more than one, and all of the described terms. For example, a reference to “at least one of ‘A’ and ‘B’” can include only ‘A’, only ‘B’, as well as both ‘A’ and ‘B’. Such references used in conjunction with “comprising” or other open terminology can include additional items. 
     The construction and arrangement of the systems and methods as shown in the various exemplary embodiments are illustrative only. Although only example embodiments have been described in detail in this disclosure, many modifications are possible (e.g., variations in sizes, dimensions, structures, shapes and proportions of the various elements, values of parameters, mounting arrangements, use of materials, colors, orientations, etc.). For example, the position of elements can be reversed or otherwise varied and the nature or number of discrete elements or positions can be altered or varied. Accordingly, such modifications are intended to be included within the scope of the present disclosure. The order or sequence of any process or method steps can be varied or re-sequenced according to alternative embodiments. Other substitutions, modifications, changes, and omissions can be made in the design, operating conditions and arrangement of the exemplary embodiments without departing from the scope of the present disclosure.