Patent Publication Number: US-11035600-B2

Title: Capacity control for chillers having screw compressors

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS 
     This application is a U.S. National Stage Application of PCT/US2017/035511, filed Jun. 1, 2017, which claims the benefit of U.S. Provisional Patent Application No. 62/355,216, filed Jun. 27, 2016, both of which are incorporated herein by reference in their entirety. 
    
    
     BACKGROUND 
     The present disclosure relates generally to the field of compressor systems for refrigeration circuits and the control thereof. More specifically, the present disclosure relates to the control of compressor systems having screw compressors. 
     Screw compressors typically include two meshing helical screws or rotors configured to compress a gas. The gas enters at a suction side of the screw compressors and moves through meshing threads of the screws as the screws rotate. The meshing threads force the gas through the compressor, and the gas exits at the end of the screws with an increased temperature and pressure. 
     SUMMARY 
     One implementation of the present disclosure is related to a compressor system for a refrigeration circuit. The compressor system includes a screw compressor and a controller. The screw compressor includes a slide valve selectively actuatable between a first position and a second position to facilitate modulating a capacity of the screw compressor between fully-loaded and fully-unloaded. The controller is communicably coupled to the slide valve. The controller is configured to receive a chilled fluid temperature setpoint for a fluid in heat transfer communication with a refrigerant of the refrigeration circuit, receive temperature data indicative of a chilled fluid temperature of the fluid, determine a difference between the chilled fluid temperature and the chilled fluid temperature setpoint, and provide one of a load command and an unload command to the slide valve based the difference between the chilled fluid temperature and the chilled fluid temperature setpoint. According to an example embodiment, the controller does not receive feedback from the screw compressor regarding a position of the slide valve. 
     Another implementation of the present disclosure is related to a method for capacity control of a chiller having a compressor. The method includes receiving, by a processing circuit, a chilled fluid temperature setpoint for a fluid in heat transfer communication with a refrigerant of the chiller; receiving, by the processing circuit, temperature data from a temperature sensor indicative of a chilled fluid temperature of the fluid; providing, by the processing circuit, a load command to a slide valve of the compressor to increase the capacity of the compressor in response to the chilled fluid temperature being greater than the chilled fluid temperature setpoint; and providing, by the processing circuit, an unload command to the slide valve to decrease the capacity of the compressor in response to the chilled fluid temperature being less than the chilled fluid temperature setpoint. According to an example embodiment, the processing circuit does not receive feedback from the compressor regarding a position of the slide valve. 
     Still another implementation of the present disclosure is related to a chiller. The chiller includes a compressor, a condenser positioned downstream of the compressor, an expansion valve positioned downstream of the condenser, an evaporator positioned downstream of the expansion valve and upstream of the compressor, and a controller. The compressor is configured to provide a refrigerant throughout the chiller. The compressor has a slide valve that is selectively actuatable to facilitate modulating a capacity of the compressor. The evaporator is configured to subject the refrigerant to a heat exchange relationship with a fluid. The controller is configured to receive a temperature setpoint for the fluid in heat transfer communication with the refrigerant, receive temperature data indicative of a temperature of the fluid, provide a load command to the slide valve of the compressor to increase the capacity of the compressor in response to the temperature of the fluid being greater than the temperature setpoint, and provide an unload command to the slide valve to decrease the capacity of the compressor in response to the temperature of the fluid being less than the temperature setpoint. According to an example embodiment, the controller does not receive feedback from the compressor regarding a position of the slide valve. 
     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  a perspective view of a building serviced by a heating, ventilation, and air conditioning system (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 illustrating an alternative implementation of the refrigeration circuit of  FIG. 2 , according to an exemplary embodiment. 
         FIG. 4  is a block diagram of a compressor control system, according to an exemplary embodiment. 
         FIG. 5  is a block diagram of control logic for a compressor control system, according to an exemplary embodiment. 
         FIG. 6  is a flow diagram of a method for capacity control of chillers having screw compressors, according to an exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to  FIG. 1 , a perspective view of a building  10  is shown. Building  10  is serviced by a heating, ventilation, and air conditioning system (HVAC) system  20 . HVAC system  20  is shown to 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  is shown to include a separate AHU  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 . 
     In some embodiments, the refrigerant in chiller  22  is 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. 
     Referring now to  FIG. 2 , a block diagram illustrating a portion of HVAC system  20  in greater detail is shown, according to an exemplary embodiment. In  FIG. 2 , chiller  22  is shown to include a refrigeration circuit  42  and a controller  100 . Refrigeration circuit  42  is shown to 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 . In some embodiments, compressor  48  is 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 . According to an exemplary embodiment, compressor  48  is a screw compressor. In some embodiments, compressor  48  is a semi-hermetic screw compressor. In other embodiments, compressor  48  is a hermitic or open screw compressor. In alternative embodiments, compressor  48  is 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. In some embodiments, cooling circuit  56  is a heat recovery circuit configured to use the heat absorbed from the refrigerant for heating applications. In other embodiments, cooling circuit  56  includes 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 . In other embodiments, cooling unit  26  may be a cooling tower. The heat exchange fluid may reject heat in cooling unit  26  and return to condenser  50  via piping  30 . 
     Still referring to  FIG. 2 , refrigeration circuit  42  is shown to include a line  62  connecting an outlet of condenser  50  to an inlet of expansion device  52 . Expansion device  52  may be configured to 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. In some embodiments, chilled fluid circuit  66  includes 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 AHU  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 . 
     As shown in  FIG. 2 , chilled fluid circuit  66  includes a chilled fluid temperature sensor  74  positioned along piping  32 . Chilled fluid temperature sensor  74  may be configured to measure a temperature T cf  of the chilled fluid (e.g., the leaving chilled liquid temperature, etc.) flowing within piping  32  between evaporator  46  and AHU  36 . As shown in  FIG. 2 , refrigeration circuit  42  includes a suction temperature sensor  76  positioned along compressor suction line  72 . Suction temperature sensor  76  may be configured to measure 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 ). As shown in  FIG. 2 , refrigeration circuit  42  includes a suction pressure sensor  78  positioned along compressor suction line  72 . Suction pressure sensor  78  may be configured to measure 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 ). As shown in  FIG. 2 , refrigeration circuit  42  includes a discharge temperature sensor  80  positioned along compressor discharge line  54 . Discharge temperature sensor  80  may be configured to measure 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 ). As shown in  FIG. 2 , refrigeration circuit  42  includes a discharge pressure sensor  82  positioned along compressor discharge line  54 . Discharge pressure sensor  82  may be configured to measure 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 ). 
     Referring now to  FIG. 3 , a refrigeration circuit  84  is shown, according to another exemplary embodiment. Refrigeration circuit  84  may be the same or similar to refrigeration circuit  42  as described with reference to  FIG. 2 , but implemented in a more general setting. For example, refrigeration circuit  84  is shown to include evaporator  46 , compressor  48 , condenser  50 , expansion device  52 , compressor discharge line  54 , line  62 , line  64 , compressor suction line  72 , suction temperature sensor  76  and suction pressure sensor  78  positioned along compressor suction line  72 , and discharge temperature sensor  80  and discharge pressure sensor  82  positioned along compressor discharge line  54 . Refrigeration circuit  84  may be implemented in a chiller (e.g., chiller  22 ) or used in a various other refrigeration systems or devices such as refrigerators, freezers, refrigerated display cases, refrigerated storage devices, product coolers, standalone air conditioners, or any other system or device that provides cooling using a vapor-compression refrigeration loop. 
     In refrigeration circuit  84 , evaporator  46  is shown absorbing heat from an airflow  90  forced through or across evaporator  46  by a fan  94 . Similarly, condenser  50  is shown rejecting heat to an airflow  92  forced through or across condenser  50  by a fan  96 . Fan  94  and fan  96  may be controlled by controller  100  to modulate the rate of heat transfer in evaporator  46  and/or condenser  50 , respectively. In some embodiments, fan  94  and/or fan  96  are variable speed fans capable of operating at multiple different speeds. Controller  100  may increase or decrease the speed of fan  94  and/or fan  96  in response to various inputs from refrigeration circuit  84  (e.g., temperature measurements, pressure measurements, etc.). 
     Refrigeration circuit  84  is shown to include a chilled fluid temperature sensor  88  positioned within airflow  90  downstream of evaporator  46 . Chilled fluid temperature sensor  88  may be configured to measure the temperature of airflow  90  after airflow  90  is chilled by evaporator  46 . Controller  100  may be configured to control operation of compressor  48  at least partially based on measurement inputs received from at least one of chilled fluid temperature sensor  88 , suction temperature sensor  76 , suction pressure sensor  78 , discharge temperature sensor  80 , and discharge pressure sensor  82 . In other embodiments, refrigeration circuit  84  exchanges heat with one or more closed fluid circuits (e.g., chilled fluid circuit  66 , cooling circuit  56 , etc.) as described with reference to  FIG. 2 . In such embodiments, controller  100  may receive a measurement of a chilled fluid temperature. 
     Controller  100  may receive measurement inputs from at least one of chilled fluid temperature sensor  74  or chilled fluid temperature sensor  88 , suction temperature sensor  76 , suction pressure sensor  78 , discharge temperature sensor  80 , and discharge pressure sensor  82 . Controller  100  may be configured to control operation of compressor  48  (e.g., a slide valve thereof, etc.) at least partially based on the measurement inputs received from at least one of chilled fluid temperature sensor  74  or chilled fluid temperature sensor  88 , suction temperature sensor  76 , suction pressure sensor  78 , discharge temperature sensor  80 , and discharge pressure sensor  82 . Controller  100  may be an embedded controller for chiller  22  configured to control the components of refrigeration circuit  42  and/or refrigeration circuit  84 . For example, controller  100  may be configured to activate/deactivate compressor  48  and open/close expansion device  52 . Controller  100  may be configured to determine thermodynamic properties of the refrigerant at various locations within refrigeration circuit  42  and/or refrigeration circuit  84  based on the measurement inputs from at least one of chilled fluid temperature sensor  74  or chilled fluid temperature sensor  88 , suction temperature sensor  76 , suction pressure sensor  78 , discharge temperature sensor  80 , and discharge pressure sensor  82 . For example, controller  100  may calculate non-measured thermodynamic properties (e.g., enthalpy, entropy, etc.) of the refrigerant in compressor suction line  72 , compressor discharge line  54 , and/or other locations within refrigeration circuit  42 . 
     Referring now to  FIG. 4 , a block diagram of a compressor control system including the controller  100  is shown, according to an exemplary embodiment. According to the exemplary embodiment shown in  FIG. 4 , compressor  48  is configured as a screw compressor having a slide valve, shown as slide valve  49 . Slide valve  49  may be selectively actuated to modulate (e.g., increase, decrease, load, unload, etc.) the capacity of compressor  48 . Controller  100  is configured to selectively actuate slide valve  49  without receiving feedback regarding the current position of slide valve  49  (e.g., the current position of slide valve  49  is unknown, the current capacity of compressor  48  is unknown, etc.), according to an exemplary embodiment. 
     As shown in  FIG. 4 , controller  100  includes 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 NFC 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.). 
     In some embodiments, communications interface  102  is configured to facilitate receiving measurement inputs from various sensors. The sensors may include, for example, chilled fluid temperature sensor  74  configured to measure the temperature of the chilled fluid at an outlet of evaporator  46 , suction pressure sensor  78  configured to measure the pressure of the refrigerant in compressor suction line  72 , discharge pressure sensor  82  configured to measure 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  88 , etc.). Communications interface  102  may receive the measurement 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 . In some embodiments, communications interface  102  is configured to facilitate transmitting load and unload commands to slide valve  49  of compressor  48  and/or receiving load/unload timer information regarding loading/unloading of compressor  48 . 
     As shown in  FIG. 4 , processing circuit  104  includes 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. 
     As shown in  FIG. 4 , memory  108  includes various modules for completing processes described herein. More particularly, memory  108  includes a temperature module  110 , a pressure module  112 , a timer module  114 , and a load module  116 . While various modules with particular functionality are shown in  FIG. 4 , it should be understood that 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. Further, it should be understood that the controller  100  may further control other processes beyond the scope of the present disclosure. 
     As shown in  FIG. 4 , temperature module  110  is coupled to (e.g., in data receiving communication with, etc.) chilled fluid temperature sensor  74 . Temperature module  110  may be configured to receive temperature data from chilled fluid temperature sensor  74 . The temperature data may be indicative of the temperature T cf  of the chilled fluid (e.g., the leaving chilled liquid temperature, etc.) flowing within piping  32  between evaporator  46  and AHU  36 . In some embodiments, temperature module  110  is configured to receive and store a chilled fluid temperature setpoint (e.g., a leaving chilled liquid setpoint, etc.). The chilled fluid temperature setpoint may be predefined and stored within memory  108  during manufacture, entered via an operator of HVAC system  20  (e.g., via a user interface device, etc.), and/or based on a desired room temperature (e.g., entered by occupants of building  10  using a thermostat, etc.). The chilled fluid temperature setpoint may indicate a desired temperature T cf  for the chilled fluid flowing within piping  32  from evaporator  46  to AHU  36  (e.g., to perform a desired cooling operation to provide a desired conditioned air temperature within building  10 , etc.). Temperature module  110  may be further configured to compare the temperature T cf  of the chilled fluid to the chilled fluid temperature setpoint to determine a difference between the temperature T cf  of the chilled fluid and the chilled fluid temperature setpoint. The difference between the temperature T cf  of the chilled fluid and the chilled fluid temperature setpoint may be used by load module  116  when controlling slide valve  49  and/or compressor  48 . 
     As shown in  FIG. 4 , pressure module  112  is coupled to (e.g., in data receiving communication with, etc.) suction pressure sensor  78 . Pressure module  112  may be configured to receive first pressure data from suction pressure sensor  78 . The first pressure data may be indicative of the pressure P suc  of the refrigerant flowing within compressor suction line  72  into the inlet of compressor  48 . As shown in  FIG. 4 , pressure module  112  is coupled to (e.g., in data receiving communication with, etc.) discharge pressure sensor  82 . Pressure module  112  may be configured to receive second pressure data from discharge pressure sensor  82 . The second pressure data may be indicative of the pressure P dis  of the refrigerant flowing out of the outlet of compressor  48  within compressor discharge line  54 . The pressure P suc  and/or the pressure P dis  may be used by load module  116  when controlling slide valve  49  and/or compressor  48 . 
     Timer module  114  may be configured to initiate and/or continue a load timer each time compressor  48  receives a load command, as described further herein. The timer module  114  may be configured to initiate and/or continue an unload timer each time compressor  48  receives an unload command, as described further herein. The load timer and/or the unload timer may be used to estimate the current position of slide valve  49  as controller  100  modulates the capacity of compressor  48 . By way of example, slide valve  49  may take a first amount of time to reach (e.g., actuate into, stroke into, etc.) a fully-loaded position (e.g., from a nominal/neutral position, from a fully-unloaded position, etc.) such that compressor  48  operates at maximum capacity. By way of another example, slide valve  49  may take a second amount of time to reach (e.g., actuate into, stroke into, etc.) a fully-unloaded position (e.g., from a nominal/neutral position, from the fully-loaded position, etc.) such that compressor  48  operates at minimum capacity. The load timer and the unload timer may be a single timer that counts positive time while compressor  48  is being loaded and negative time when the compressor  48  is being unloaded (e.g., zero time may represent a neutral, nominal, half-way position of slide valve  49 ; zero or a minimum threshold may represent a fully-unloaded position; a maximum threshold may represent a fully-loaded position; etc.). The load timer and/or the unload timer may be used by load module  116  when controlling slide valve  49  and/or compressor  48 . 
     In some embodiments, the first amount of time for slide valve  49  to reach the fully-loaded position is greater than (e.g., takes a greater amount of time, etc.) than the second amount of time for slide valve  49  to reach the fully-unloaded position (e.g., slide valve  49  is spring biased towards the fully-unloaded position, etc.). In other embodiments, the second amount of time for slide valve  49  to reach the fully-unloaded position is greater than (e.g., takes a greater amount of time, etc.) than the first amount of time for slide valve  49  to reach the fully-loaded position (e.g., slide valve  49  is spring biased towards the fully-loaded position, etc.). In still other embodiments, the first amount of time for slide valve  49  to reach the fully-loaded position and the second amount of time for slide valve  49  to reach the fully-unloaded position are the same. 
     As shown in  FIG. 4 , load module  116  is coupled to (e.g., in data receiving and/or command transmitting communication with, etc.) compressor  48  and/or slide valve  49  thereof. Load module  116  may be configured to transmit a load command and/or an unload command to slide valve  49  of compressor  48  based on at least one of (i) the difference between the temperature T cf  of the chilled fluid and the chilled fluid temperature setpoint, (ii) the pressure P suc , (iii) the pressure P dis , (iv) the load timer, and (iv) the unload timer. 
     According to an exemplary embodiment, load module  116  is configured to provide a load command to slide valve  49  (e.g., increasing the size of the inlet of compressor  48 , etc.) to increase the capacity of compressor  48  in response to the temperature T cf  of the chilled fluid within piping  32  being greater than the chilled fluid temperature setpoint. By way of example, increasing the capacity of compressor  48  may facilitate compressor  48  in increasing the circulation (e.g., flow rate, mass flow rate, volume flow rate, etc.) of the refrigerant through the refrigeration circuit  42 . (or refrigeration circuit  84 ). Increasing the circulation of the refrigerant may increase the amount of heat removed from the fluid of chilled fluid circuit  66  flowing through evaporator  46 , thereby reducing the temperature T cf  of the chilled fluid. 
     In some embodiments, load module  116  is configured to continue providing the load command until at least one of (i) the temperature T cf  of the chilled fluid decreases such that the temperature T cf  is equal to or approximately equal to (e.g., within a predetermined range of, etc.) the chilled fluid temperature setpoint and (ii) the load timer reaches a load time threshold. By way of example, load module  116  may be configured to stop providing the load command to slide valve  49  and stop the load timer in response to the temperature T cf  of the chilled fluid decreasing such that the temperature T cf  is equal to or approximately equal to the chilled fluid temperature setpoint (e.g., the capacity of compressor  48  does not need to be increased further to provide the chilled fluid at the chilled fluid temperature setpoint, etc.). 
     By way of another example, load module  116  may be configured to stop the load timer and continue providing the load command for a predetermined amount of time (e.g., five seconds, thirty seconds, one minute, etc.) in response to the load timer reaching the load time threshold indicating that compressor  48  is fully-loaded (e.g., slide valve  49  is positioned in a fully-open position, etc.). The load time threshold (e.g., the elapsed time for slide valve  49  to move or stroke from a fully-closed position to a fully-open position, etc.) may be predefined and stored within load module  116  based on design characteristics of compressor  48  and/or slide valve  49 . Load module  116  may be configured to stop providing the load command to slide valve  49  after the predetermined amount of time has elapsed, but compressor  48  may continue to operate at full-load (e.g., as long as the temperature T cf  of the chilled fluid has not yet decreased such that the temperature T cf  is equal to or approximately equal to the chilled fluid temperature setpoint, slide valve  49  remains positioned in the fully-open position, etc.). According to an exemplary embodiment, load module  116  continues to provide the load command for the predetermined amount of time after the load timer reaches the load time threshold to prevent and/or reduce potential drift of slide valve  49  and/or the capacity of compressor  48 . 
     In some embodiments, load module  116  is configured to determine a load limit for compressor  48  based on the pressure P suc  of the refrigerant entering compressor  48  and the pressure P dis  of the refrigerant exiting compressor  48 . Load module  116  may be configured to provide the load command to slide valve  49  in such a way that operation of compressor  48  (e.g., operating characteristics thereof, etc.) does not exceed the load limit (e.g., the load command is stopped in response to the load limit being reached, etc.). Limiting the operation of compressor  48  within the load limit may prevent tripping a fault threshold. The fault threshold may be configured to shut compressor  48  down and/or limit operation thereof in response to operating conditions becoming too extreme (e.g., to protect compressor  48  and/or other components of chiller  22 , etc.). 
     According to an exemplary embodiment, load module  116  is configured to provide an unload command to slide valve  49  (e.g., reducing the size of the inlet of compressor  48 , etc.) to decrease the capacity of compressor  48  in response to the temperature T cf  of the chilled fluid within piping  32  being less than the chilled fluid temperature setpoint. By way of example, decreasing the capacity of compressor  48  may facilitate compressor  48  in decreasing the circulation (e.g., flow rate, mass flow rate, volume flow rate, etc.) of the refrigerant through the refrigeration circuit  42  (or refrigeration circuit  84 ). Decreasing the circulation of the refrigerant may decrease the amount of heat removed from the fluid of chilled fluid circuit  66  flowing through evaporator  46 , thereby increasing the temperature T cf  of the chilled fluid. 
     In some embodiments, load module  116  is configured to continue providing the unload command until at least one of (i) the temperature T cf  of the chilled fluid increases such that the temperature T cf  is equal to or approximately equal to (e.g., within a predetermined range of, etc.) the chilled fluid temperature setpoint and (ii) the unload timer reaches an unload time threshold. By way of example, load module  116  may be configured to stop providing the unload command to slide valve  49  and stop the unload timer in response to the temperature T cf  of the chilled fluid increasing such that the temperature T cf  is equal to or approximately equal to the chilled fluid temperature setpoint (e.g., the capacity of compressor  48  does not need to be decreased further to provide the chilled fluid at the chilled fluid temperature setpoint, etc.). 
     By way of another example, load module  116  may be configured to stop the unload timer, stop providing the unload command, and take compressor  48  offline in response to the unload timer reaching the unload time threshold indicating that compressor  48  is fully-unloaded (e.g., slide valve  49  is positioned in the fully-closed position, etc.). The unload time threshold (e.g., the elapsed time for slide valve  49  to move or stroke from the fully-open position to the fully-closed position, etc.) may be predefined and stored within load module  116  based on design characteristics of compressor  48  and/or slide valve  49 . According to an exemplary embodiment, load module  116  takes compressor  48  offline after the unload timer reaches the unload time threshold to conserve energy and since compressor  48  may not circulate the refrigerant when full-unloaded. Load module  116  may bring compressor  48  back online and provide the load command once the temperature T cf  of the chilled fluid exceeds the chilled fluid temperature setpoint. 
     Referring now to  FIG. 5 , a block diagram of control logic for a compressor control system is shown, according to an exemplary embodiment. According to the exemplary embodiment shown in  FIG. 5 , the unit (e.g., HVAC system  20 , etc.) includes two systems (e.g., two compressor systems, etc.). In other embodiments, the unit includes more or fewer systems (e.g., one compressor system, three compressor systems, etc.). At process  502 , a leaving chilled liquid temperature is received (e.g., by controller  100 , from chilled fluid temperature sensor  74 , etc.). At process  504 , a leaving chilled liquid setpoint is received (e.g., by controller  100 , from an operator, predefined in memory  108 , etc.). At process  506 , the leaving chilled liquid temperature and the leaving chilled liquid setpoint are compared (e.g., by controller  100 , using fuzzy logic, etc.). At process  508 , a difference between the leaving chilled liquid temperature and the leaving chilled liquid setpoint is determined (e.g., by controller  100 , etc. 
     At process  510   a , a first load/unload time accumulator sends a first timer signal (e.g., to controller  100 , etc.) regarding loading time and/or unloading time of a first compressor (e.g., a first compressor  48 , etc.) of a first system. At process  510   b , a second load/unload time accumulator sends a second timer signal regarding loading time and/or unloading time of a second compressor (e.g., a second compressor  48 , etc.) of a second system. At process  512 , the difference between the leaving chilled liquid temperature and the leaving chilled liquid setpoint, the first timer signal, and/or the second timer signal are interpreted (e.g., analyzed, by controller  100 , etc.). At process  514  and process  516 , at least one of a unit load command and a unit unload command are provided (e.g., to a system controller, a subcomponent of controller  100 , load module  116 , etc.). At process  518 , the at least one of the unit load command and the unit unload command are received and interpreted (e.g., by the system controller, etc.). 
     At process  520   a  and  522   a , a first system load command and/or a first system unload command are provided to the first system based on the unit load command and/or the unit unload command. At process  524   a  and  526   a , a first suction pressure and a first discharge pressure are received (e.g., from suction pressure sensor  78  and discharge pressure sensor  82 , etc.). At process  528   a , a first load limit is determined for the first system based on the first suction pressure and the first discharge pressure and compared to the first system load command and/or the first system unload command. At process  530   a , the first system load command is provided to a first slide valve (e.g., slide valve  49 , etc.) of the first compressor and the first load time accumulator begins/continues a first load timer. At process  532   a , the first slide valve performs an action (e.g., repositions, moves towards a fully-open position, etc.) according to the first system load command to increase the capacity of the first compressor. At process  534   a , the first system unload command is provided to the first slide valve of the first compressor and the first unload time accumulator begins/continues a first unload timer. At process  536   a , the first slide valve performs an action (e.g., repositions, moves towards a fully-closed position, etc.) according to the first system unload command to decrease the capacity of the first compressor. 
     At process  520   b  and  522   b , a second system load command and/or a second system unload command are provided to the second system based on the unit load command and/or the unit unload command. At process  524   b  and  526   b , a second suction pressure and a second discharge pressure are received (e.g., from suction pressure sensor  78  and discharge pressure sensor  82 , etc.). At process  528   b , a second load limit is determined for the second system based on the second suction pressure and the second discharge pressure and compared to the second system load command and/or the second system unload command. At process  530   b , the second system load command is provided to a second slide valve (e.g., slide valve  49 , etc.) of the second compressor and the second load time accumulator begins/continues a second load timer. At process  532   b , the second slide valve performs an action (e.g., repositions, moves towards a fully-open position, etc.) according to the second system load command to increase the capacity of the second compressor. At process  534   b , the second system unload command is provided to the second slide valve of the second compressor and the second unload time accumulator begins/continues a second unload timer. At process  536   b , the second slide valve performs an action (e.g., repositions, moves towards a fully-closed position, etc.) according to the second system unload command to decrease the capacity of the second compressor. 
     Referring now to  FIG. 6 , a method  600  for capacity control of chillers having screw compressors where a position of a slide valve thereof is unknown (e.g., not directly known, etc.) is shown, according to an exemplary embodiment. At step  602 , a controller (e.g., the controller  100 , etc.) is configured to receive a chilled fluid temperature setpoint. In some embodiments, the chilled fluid temperature setpoint is predefined and stored within the controller during manufacture. In some embodiments, the chilled fluid temperature setpoint is entered by an operator. In some embodiments, the chilled fluid temperature setpoint is determined by the controller based on a desired temperature entered by an occupant of a building/room (e.g., via a thermostat, etc.). The chilled fluid temperature setpoint may indicate a desired temperature for a chilled fluid flowing within a chilled fluid circuit (e.g., chilled fluid circuit  66 , etc.) in thermal communication with a refrigerant of a refrigeration circuit (e.g., refrigeration circuit  42 , through evaporator  46 , etc.) of the chiller (e.g., chiller  22 , etc.) having a screw compressor (e.g., compressor  48 , etc.). The chilled fluid may be provided to an AHU (e.g., AHU  36 , etc.) to perform a desired cooling operation to provide a desired conditioned air temperature within a building/room. 
     At step  604 , the controller is configured to receive temperature data indicative of a chilled fluid temperature of the chilled fluid of the chilled fluid circuit from a temperature sensor (e.g., chilled fluid temperature sensor  74 , etc.). At step  606 , the controller is configured to determine a difference between the chilled fluid temperature and the chilled fluid temperature setpoint. At step  608 , the controller is configured to determine whether the chilled fluid temperature is greater than the chilled fluid temperature setpoint. The controller is configured to return to step  602  in response to the chilled fluid temperature being equal to or approximately equal to (e.g., within a predefined range of, etc.) the chilled fluid temperature setpoint (i.e., the capacity of the screw compressor does not need to be adjusted as the temperature of the chilled fluid is at or near the setpoint). 
     At step  610 , the controller is configured to transmit a load command to the screw compressor in response to the chilled fluid temperature being greater than the chilled fluid temperature setpoint (e.g., to increase the capacity of the screw compressor to thereby decrease the temperature of the chilled fluid, etc.). At step  612 , the controller is configured to transmit an unload command to the screw compressor in response to the chilled fluid temperature being less than the chilled fluid temperature setpoint (e.g., to decrease the capacity of the screw compressor to thereby increase the temperature of the chilled fluid, etc.). 
     At step  614 , the controller is configured to receive first pressure data indicative of a suction pressure of the refrigerant entering the screw compressor from a first pressure sensor (e.g., suction pressure sensor  78 , etc.). At step  616 , the controller is configured to receive second pressure data indicative of a discharge pressure of the refrigerant exiting the screw compressor from a second pressure sensor (e.g., discharge pressure sensor  82 , etc.). At step  618 , the controller is configured to determine a load limit of the screw compressor based on the suction pressure and the discharge pressure of the refrigerant. At step  620 , the controller is configured to operate a load control scheme (steps  622 - 634 ) if the load command was transmitted to the screw compressor or operate a unload control scheme (steps  636 - 646 ) if the unload command was transmitted to the screw compressor. 
     At step  622 , the controller is configured to provide the load command to a slide valve (e.g., slide valve  49 , etc.) of the screw compressor to load the screw compressor (e.g., actuate the slide valve to increase the inlet opening of the screw compressor to increase the refrigerant circulation, etc.). The load command may be provided so long as the load of the screw compressor does not exceed the load limit (e.g., to prevent a fault threshold from being reached, etc.). At step  624 , the controller is configured to start a load timer or continue a previously stopped load timer. At  626 , the controller is configured to determine whether the chilled fluid temperature is equal to or approximately equal to the chilled fluid temperature setpoint (i.e., has the chilled fluid temperature dropped to the chilled fluid temperature setpoint since providing the load command to the slide valve). If the chilled fluid temperature is equal to or approximately equal to the chilled fluid temperature setpoint, the controller is configured to stop providing the load command to the slide valve and stop the load timer (e.g., the screw compressor continues to operate at the current state, the slide valve remains in its current position, etc.) (step  628 ) and may return to step  602 . If the chilled fluid temperature is not equal to or approximately equal to the chilled fluid temperature setpoint, the controller is configured to proceed to step  630 . 
     At step  630 , the controller is configured to determine whether the load timer has reached a load time threshold. The load time threshold (e.g., an elapsed time for the slide valve to move or stroke from a fully-closed position to a fully-open position, etc.) may be predefined and stored within the controller based on design characteristics of the screw compressor and/or the slide valve. If the chilled temperature setpoint and the load time threshold are both not reached, the controller returns to step  622  to continue providing the load command to the slide valve until at least one of (i) the chilled fluid temperature setpoint is reached (step  626 ) and (ii) the load timer is reached (step  630 ). If the load time threshold is reached prior to the chilled fluid temperature decreasing to satisfy the chilled fluid temperature setpoint, the controller is configured to continue providing the load command for a predetermined period of time and stop the load timer (step  632 ). Reaching the load time threshold may indicate that the slide valve is fully-open (i.e., the screw compressor is at maximum capacity, fully-loaded). The load command may be provided after the load timer reaches the load time threshold to prevent and/or reduce potential drift of the slide valve and/or the capacity of the screw compressor. At step  634 , the controller is configured to stop providing the load command to the slide valve such that the screw compressor operates at its current capacity (e.g., the maximum capacity, the load limit capacity, the fully-loaded capacity, etc.) and return to step  602 . 
     At step  636 , the controller is configured to provide the unload command to the slide valve of the screw compressor to unload the screw compressor (e.g., actuate the slide valve to decrease the inlet opening of the screw compressor to decrease the refrigerant circulation, etc.). The unload command may be provided so long as the load of the screw compressor does not exceed the load limit (e.g., to prevent a fault threshold from being reached, etc.). At step  638 , the controller is configured to start an unload timer or continue a previously stopped unload timer (or subtract from the load timer). At  640 , the controller is configured to determine whether the chilled fluid temperature is equal to or approximately equal to the chilled fluid temperature setpoint (i.e., has the chilled fluid temperature increased to the chilled fluid temperature setpoint since providing the unload command to the slide valve). If the chilled fluid temperature is equal to or approximately equal to the chilled fluid temperature setpoint, the controller is configured to stop providing the unload command to the slide valve and stop the unload timer (e.g., the screw compressor continues to operate at the current state, the slide valve remains in its current position, etc.) (step  642 ) and may return to step  602 . If the chilled fluid temperature is not equal to or approximately equal to the chilled fluid temperature setpoint, the controller is configured to proceed to step  644 . 
     At step  644 , the controller is configured to determine whether the unload timer has reached an unload time threshold. The unload time threshold (e.g., an elapsed time for the slide valve to move or stroke from a fully-open position to a fully-closed position, etc.) may be predefined and stored within the controller based on design characteristics of the screw compressor and/or the slide valve. If the chilled temperature setpoint and the unload time threshold are both not reached, the controller returns to step  636  to continue providing the load command to the slide valve until at least one of (i) the chilled fluid temperature setpoint is reached (step  640 ) and (ii) the unload timer is reached (step  644 ), If the unload time threshold is reached prior to the chilled fluid temperature increasing to satisfy the chilled fluid temperature setpoint, the controller is configured to stop the unload timer, stop providing the unload command, and take the compressor offline (step  646 ) and return to step  602 . Reaching the unload time threshold may indicate that the slide valve is fully-closed (e.g., the screw compressor is at minimum capacity, zero-load, etc.). 
     The construction and arrangement of the systems and methods as shown in the various exemplary embodiments are illustrative only. Although only a few 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, all 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. 
     The present disclosure contemplates methods, systems and program products on any machine-readable media for accomplishing various operations. The embodiments of the present disclosure can be implemented using existing computer processors, or by a special purpose computer processor for an appropriate system, incorporated for this or another purpose, or by a hardwired system. Embodiments within the scope of the present disclosure include program products comprising machine-readable media for carrying or having machine-executable instructions or data structures stored thereon. Such machine-readable media can be any available media that can be accessed by a general purpose or special purpose computer or other machine with a processor. By way of example, such machine-readable media can comprise RAM, ROM, EPROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other medium which can be used to carry or store desired program code in the form of machine-executable instructions or data structures and which can be accessed by a general purpose or special purpose computer or other machine with a processor. Combinations of the above are also included within the scope of machine-readable media. Machine-executable instructions include, for example, instructions and data which cause a general purpose computer, special purpose computer, or special purpose processing machines to perform a certain function or group of functions. 
     Although the figures show a specific order of method steps, the order of the steps may differ from what is depicted. Also two or more steps can be performed concurrently or with partial concurrence. Such variation will depend on the software and hardware systems chosen and on designer choice. All such variations are within the scope of the disclosure. Likewise, software implementations could be accomplished with standard programming techniques with rule based logic and other logic to accomplish the various connection steps, processing steps, comparison steps and decision steps.