Patent Publication Number: US-11378314-B2

Title: Air cooled chiller with heat recovery

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 14/655,583, entitled “AIR COOLED CHILLER WITH HEAT RECOVERY,” filed Jun. 25, 2015, which is expected to be patented as U.S. Pat. No. 10,401,068, and is a U.S. National Stage Application of International Application No. PCT/US2014/011510, filed on Jan. 14, 2014, which claims the benefit of U.S. Provisional Application No. 61/752,821, filed on Jan. 15, 2013, all of which are incorporated by reference in their entireties for all purposes. 
    
    
     BACKGROUND 
     The present disclosure relates generally to refrigeration systems employed for chiller applications, and, more specifically, to chiller systems that provide heat recovery. 
     Certain refrigeration and air conditioning systems rely on chillers to reduce the temperature of a process fluid, typically water. In such applications, the chilled water may be passed through downstream equipment, such as air handlers, to cool other fluids, such as air in a building. In typical chillers, the process fluid is cooled by an evaporator that absorbs heat from the process fluid by evaporating refrigerant. The refrigerant is then compressed by a compressor and transferred to a condenser. In the condenser, the refrigerant is cooled, typically by air or water flows, and recondensed into a liquid. Air cooled condensers typically comprise one or more condenser coils and one or more fans that induce airflow over the coils. Some systems may employ economizers to improve performance. In systems with flash tank economizers, the condensed refrigerant exiting the condenser coils is directed to a flash tank where the liquid refrigerant at least partially evaporates. The vapor may be extracted from the flash tank and returned to the compressor, while liquid refrigerant from the flash tank is directed to the evaporator, closing the refrigeration loop. In systems with heat exchanger economizers, the condensed refrigerant exiting the condenser coils is split into two flow streams that flow on the two sides of a heat exchanger. One of the two flow streams evaporates and cools the second stream. The flow stream that evaporates flows to the compressor while the other stream flows to the evaporator, closing the refrigeration loop. 
     In some conventional air-cooled chiller designs, heat recovery heat exchangers (HRHXs) may be utilized to provide auxiliary heating of water or other process fluids for use in the building. In such systems, the compressed refrigerant flows through the HRHX before entering the condenser in order to transfer heat to fluid that is circulated through the HRHX. If no fluid is circulated through the HRHX, then the refrigeration system may function as a typical air-cooled chiller. Unfortunately, as the demand for heat recovery increases, the refrigerant exiting the HRHX may become more condensed. This may decrease the amount of refrigerant vapor available for heat transfer through the condenser. As a result, the amount of liquid refrigerant in the condenser may increase, while the amount of liquid refrigerant in the evaporator decreases. This could lead to a loss of liquid refrigerant level in the evaporator, causing the refrigeration system to trip due to low suction pressure. In addition, as the desired heat recovery load increases, the system may be difficult to control using conventional chiller controls. For example, as the demand for heat recovery increases, conventional chiller control models may output condenser fans speeds that are below desired levels for promoting good heat transfer within the condenser. There is a need, therefore, for improved techniques for controlling chiller applications that include heat recovery systems. 
    
    
     
       DRAWINGS 
         FIG. 1  is an illustration of an exemplary embodiment of a commercial heating ventilating, air conditioning and refrigeration (HVAC&amp;R) system that includes an air cooled refrigeration system in accordance with aspects of the present techniques; 
         FIG. 2  is a diagrammatical representation of an exemplary HVAC&amp;R system in accordance with the present techniques. 
         FIG. 3  is a table illustrating various presently contemplated modes of operation of the system of  FIG. 2 , and how certain components may be controlled in the various modes; 
         FIG. 4  is a flowchart of a method for responding to various heat recovery loads on the system of  FIG. 2 ; 
         FIG. 5  is a flowchart of a method for operating the system of  FIG. 2  in intermediate heat recovery mode; 
         FIG. 6  is a diagrammatical representation of an exemplary HVAC&amp;R system in accordance with the present techniques; and 
         FIG. 7  is a diagrammatical representation of an exemplary HVAC&amp;R system including a heat exchanger economizer in accordance with the present techniques. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure is directed to systems and methods for controlling an air cooled chiller with auxiliary heat recovery. The system may include, among other things, a compressor, condenser, expansion device, economizer, and evaporator for circulating refrigerant, as well as a heat recovery heat exchanger that transfers heat from the refrigerant to heat a process fluid. A controller controls the expansion device and a condenser fan based on sensor feedback in order to provide a desired amount of heat recovery. The system may be particularly beneficial in chillers employing microchannel air-cooled condenser that have a relatively small interior refrigerant volume and shell side evaporators that have a relatively large interior refrigerant volume. According to certain embodiments, the techniques described herein may be designed to provide smooth control from zero to 100% heat recovery from the refrigeration system. 
       FIG. 1  depicts an exemplary application for a refrigeration system. 
     Such systems, in general, may be applied in a range of settings, both within the HVAC&amp;R field and outside of that field. The refrigeration systems may provide cooling to data centers, electrical devices, freezers, coolers, or other environments through vapor-compression refrigeration, absorption refrigeration, or thermoelectric cooling. In presently contemplated applications, however, refrigeration systems may be used in residential, commercial, light industrial, industrial, and in any other application for heating or cooling a volume or enclosure, such as a residence, building, structure, and so forth. Moreover, the refrigeration systems may be used in industrial applications, where appropriate, for basic refrigeration and heating of various fluids. 
       FIG. 1  illustrates an exemplary application, in this case an HVAC&amp;R system for building environmental management that may employ heat exchangers. A building  10  is cooled by a system that includes a chiller  12  and a boiler  14 . As shown, chiller  12  is disposed on the roof of building  10  and boiler  14  is located in the basement; however, the chiller and boiler may be located in other equipment rooms or areas next to the building. Chiller  12  is an air cooled or water cooled device that implements a refrigeration cycle to cool water. Chiller  12  is housed within a single structure that includes a refrigeration circuit and associated equipment such as pumps, valves, and piping. For example, chiller  12  may be a single package rooftop unit. Boiler  14  is a closed vessel in which water is heated. The water from chiller  12  and boiler  14  is circulated through building  10  by water conduits  16 . Water conduits  16  are routed to air handlers  18 , located on individual floors and within sections of building  10 . 
     Air handlers  18  are coupled to ductwork  20  that is adapted to distribute air between the air handlers and may receive air from an outside intake (not shown). Air handlers  18  include heat exchangers that circulate cold water from chiller  12  and hot water from boiler  14  to provide heated or cooled air. Fans, within air handlers  18 , draw air through the heat exchangers and direct the conditioned air to environments within building  10 , such as rooms, apartments, or offices, to maintain the environments at a designated temperature. A control device, shown here as including a thermostat  22 , may be used to designate the temperature of the conditioned air. Control device  22  also may be used to control the flow of air through and from air handlers  18 . Other devices may, of course, be included in the system, such as control valves that regulate the flow of water and pressure and/or temperature transducers or switches that sense the temperatures and pressures of the water, the air, and so forth. Moreover, control devices may include computer systems that are integrated with or separate from other building control or monitoring systems, and even systems that are remote from the building. 
       FIG. 2  schematically depicts an embodiment of chiller  12 , which incorporates a heat recovery system and may be controlled by a controller  24 . As discussed further below, the heat recovery system may provide an auxiliary function that heats a liquid using some or all of the heat normally rejected to the environment by chiller  12 . Chiller  12  includes a cooling fluid loop  23  that circulates a cooling fluid, such as chilled water, an ethylene glycol-water solution, brine, or the like, to a cooling load, such as a building, piece of equipment, or environment. For example, cooling fluid loop  23  may circulate the cooling fluid to water conduits  16  shown in  FIG. 1 . In certain embodiments, the cooling fluid may circulate within the cooling fluid loop  23  to a cooling load, such as a research laboratory, computer room, office building, hospital, molding and extrusion plant, food processing plant, industrial facility, machine or any other environments or devices in need of cooling. 
     Warm fluid from cooling fluid loop  23  enters an evaporator  26  and is cooled, generating chilled fluid that can be returned to the cooling load. In cooling the fluid, evaporator  26  transfers heat from the cooling fluid loop  23  to refrigerant flowing within a closed refrigerant loop  27 . The refrigerant may be any fluid that absorbs and extracts heat. For example, the refrigerant may be a hydrofluorocarbon (HFC) based R-410A, R-407C, or R-134a, or it may be carbon dioxide (R-744) or ammonia (R-717) or hydrofluoroolefin (HFO) based. As the refrigerant flows through evaporator  26 , the refrigerant is vaporized. The vaporized refrigerant then exits evaporator  26  and flows through a suction line  28  into a compressor system  30 , which may be representative of one or more compressors. The refrigerant is compressed in compressor system  30  and exits through one or more compressor discharge lines  32 . 
     The compressed refrigerant then flows through a heat recovery heat exchanger (HRHX)  34  of a heat recovery system  35 . Heat recovery system  35  includes HRHX  34  and a heat recovery fluid loop  37  that circulates a heat recovery fluid, such as water or brine, through HRHX  34 . As the heat recovery fluid flows through HRHX  34 , the heat recovery fluid absorbs heat from the refrigerant flowing through HRHX  34  to produce warmed heat recovery fluid. According to certain embodiments, the warmed heat recovery fluid may be circulated within the building  10  ( FIG. 1 ) to provide auxiliary heating of water or another liquid for use in the building  10 . 
     From HRHX  34 , the refrigerant then travels through line  36  of refrigerant loop  27  and flows through condenser  38  where the refrigerant is further cooled and condensed to a liquid. The condensed refrigerant exits condenser  38  through liquid line  40  of refrigerant loop  27 , which directs the refrigerant through an expansion valve  42  to a flash tank  44 . According to certain embodiments, the expansion valve  42  may be a thermal expansion valve or electronic expansion valve that is operated by controller  24  to vary refrigerant flow in response to suction superheat, evaporator liquid level, or other parameters. According to certain embodiments, an economizing heat exchanger could be used instead of the flash tank  44 . Within flash tank  44 , the liquid phase refrigerant may separate from the vapor phase refrigerant and collect within a lower portion of flash tank  44 . The liquid phase refrigerant may then exit flash tank  44  and flow through an orifice  46  to evaporator  26 , completing the cycle. 
     The vapor phase refrigerant exits flash tank  44  through an economizer line  49  that directs the vapor phase refrigerant to compressor system  30 . An economizer valve  48  located in economizer line  49  may be employed to control the return of refrigerant vapor to the compressor system  30 . Through economizer line  49 , the refrigerant vapor exiting the flash tank  44 , which is at a higher pressure than the refrigerant vapor entering the compressor system  30  from the evaporator  26 , may be introduced into the compressor system  30 . The compression of the higher pressure refrigerant vapor from the flash tank  44  may increase the efficiency and capacity of the refrigeration system. While economizers are typically used with screw-type compressors, similar configurations may be employed with other compressor configurations, such as reciprocating, scroll, or multistage centrifugal compressors, for example. Further, in other embodiments, flash tank  44  and economizer line  49  may be omitted so that all refrigerant exiting condenser  38  flows to evaporator  26 . Further, in other embodiments, the flash tank  44  may be replaced by a heat exchanger economizer  71 , as illustrated in  FIG. 7 . 
     As shown in  FIG. 2 , evaporator  26  is a shell and tube evaporator where the refrigerant flows through the shell side of the evaporator while the fluid to be cooled flows through tubes within the evaporator. According to certain embodiments, evaporator  26  may be a falling film evaporator, flooded evaporator, or a hybrid of a falling film and flooded evaporator. Further, in certain embodiments, evaporator  26  could be a shell and tube evaporator where the refrigerant flows through the tubes within the evaporator while the fluid to be cooled flows through the shell side. In yet other embodiments, evaporator  26  could be a plate heat exchanger where the refrigerant and fluid to be cooled flows in channels formed by closely located plates. Further, in certain embodiments, condenser  38  may be an air cooled, microchannel condenser. In these embodiments, the refrigerant may be circulated through microchannel tubes of the condenser, and thus, the condenser may have a relatively small refrigerant volume compared to refrigerant volume available in the shell side of the evaporator. The relatively small refrigerant volume in the condenser with respect to the evaporator may allow the refrigeration system to maintain an appropriate level of liquid refrigerant in evaporator  26 , even when the condenser  38  is filled with primarily liquid refrigerant. Such a condition may occur when a demand for heat recovery is very high (e.g., near 100% of the chiller heat rejection). In these situations, the refrigerant exiting HRHX  34  may be mostly or completely condensed and accordingly, condenser  38  may receive primarily liquid phase refrigerant. 
     In the illustrated embodiment, a temperature sensor  50  and a pressure transducer  52  are disposed in the liquid line  40  that extends between condenser  38  and flash tank  44 . As summarized below, a temperature and pressure monitored by these sensors  50  and  52  may be used by controller  24  to calculate the amount of subcooling for the refrigerant exiting condenser  38 . Similarly, a temperature sensor  54  and a pressure transducer  56  are located in line  36 , which extends between HRHX  34  and condenser  38 . The temperature and pressure monitored by these sensors  54  and  56  may be used by controller  24  to determine the amount of subcooling for the refrigerant exiting HRHX  34 . Heat recovery system  35  also includes another temperature sensor  58  that measures the temperature of the heat recovery fluid exiting HRHX  34 . Further, a pressure transducer  59  disposed in compressor discharge lines  32  provides a pressure measurement that may be used to operate certain controls of the refrigeration system. 
     As shown in  FIG. 2 , HRHX  34  uses a portion of the heat normally rejected to the environment through coils  38  for auxiliary heating functions (e.g., heating water or other fluids for use in building  10 ). Accordingly, the inclusion of heat recovery system  35  in chiller  12  allows chiller  12  to both cool a process fluid for circulation through cooling fluid loop  23  and to heat a heat recovery fluid for circulation through heat recovery loop  37 . This may be especially useful for providing simultaneous heating and cooling for hotels, hospitals, process industries, and other applications having multiple demands for both heating and cooling. 
     Although the HRHX  34  may be used to heat any suitable heat recovery fluid pumped therethrough, the following discussion is directed to embodiments of the refrigeration system in the context of heating water for use in a building (e.g., building  10 ). In these embodiments, water is pumped through HRHX  34  by a pump  60 , and the refrigerant flowing through the HRHX  34  heats the water to a desired temperature. Controller  24  governs operation of a motor  62  that drives one or more condenser fans  63  at an appropriate fan speed. Controller  24  also may regulate the opening of expansion valve  42  to an appropriate position based on a desired amount of heat recovery for the auxiliary heating function. 
     Chiller  12  also includes an optional heat recovery bypass valve  64  and a condenser bypass valve  66  that may be opened or closed electronically by controller  24  in response to a given heat recovery demand on the system. For example, when auxiliary heat is not desired, bypass valve  64  may be opened to direct the refrigerant exiting compressor through bypass line  65  to line  36 , allowing the refrigerant to bypass heat recovery system  35 . In another example, when heat recovery system  35  is operating at or close to full capacity, bypass valve  66  may be opened to direct the refrigerant exiting HRHX  34  to expansion valve  42 , allowing the refrigerant to bypass condenser  38 . In certain modes of operation, a three-way heat recovery valve  68  may be opened to regulate the temperature of water flowing through HRHX  34 . For example, valve  68  may be placed in a recycle position where heated water exiting HRHX  34  is re-circulated through HRHX  34  to increase the heat transferred to the water. When the desired water temperature is achieved, valve  68  may then be placed in a building return position where the heated water exiting HRHX  34  is returned to the building to provide auxiliary heating. The chiller  12  may also include an optional valve  69  between the heat recovery heat exchanger  34  and the condenser  38 . This optional valve  69  could be controlled to ensure two-phase refrigerant flow in order to prevent the condenser  38  from filling with refrigerant liquid, which can result in low suction pressure and other operational problems. At that same time, pressure drop through the optional valve  69  should not be too high to ensure adequate flow of liquid through valve  42 . This optional valve  69  may be desirable depending on the internal volume of condenser  38  compared to the refrigerant charge. That is, the optional valve  69  may be deleted if the internal volume is small enough to allow condenser  38  to fill completely with refrigerant liquid without operational problems. 
     The operation of valves  64 ,  66 ,  68 , and  69 , as well as other components, such as valves  42  and  48  and motor  62 , may be governed by controller  24  to achieve a relatively accurate, continuous, and smooth control of the system for a desired range of zero to 100% heat recovery. That is, controller  24  may control expansion valve  42  and the condenser fan speed (via motor  62 ) such that a desired amount of heat from the refrigerant may be recovered between the compressor system  30  and the condenser  38 . Depending on the heat recovery load, controller  24  may operate in different modes, described in detail below, for controlling the various components. 
     It should be noted that although one HRHX  34  is included in the illustrated refrigeration system, in other embodiments, multiple HRHXs may be included in heat recovery system  35  to provide auxiliary heating to multiple applications. The multiple HRHXs may be connected in series, in parallel, or a combination thereof and may circulate multiple heat recovery fluids. In these embodiments, the heat recovery system  35  may include multiple pumps  60  and/or multiple three-way heat recovery valves  68  that may be operated independently of one another via controller  24  to supply water, or other heat recovery fluids, at desired temperatures to multiple applications with one or more desired heating loads. 
     Controller  24  may execute hardware or software control algorithms to regulate operation of chiller  12  and the associated heat recovery system  35 . According to exemplary embodiments, controller  24  may include an analog to digital (A/D) converter, one or more microprocessors or general or special purpose computers, a non-volatile memory, memory circuits, and an interface board. For example, the controller may include memory circuitry for storing programs and control routines and algorithms implemented for control of the various system components, such as fan motor  62  or expansion valve  42  between condenser  38  and flash tank  44 . Controller  62  also includes, or is associated with, input/output circuitry for receiving sensed signals from input sensors  50 ,  52 ,  54 ,  56 , and  58 , and interface circuitry for outputting control signals for valves  42 ,  48 ,  64 ,  66 ,  68 ,  69 , and motor  62 . For example, the controller will also typically control, for example, valving for economizer line  49 , speed and loading of compressor  30 , and so forth, and the memory circuitry may store set points, actual values, historic values and so forth for any or all such parameters. Other devices may, of course, be included in the system, such as additional pressure and/or temperature transducers or switches that sense temperatures and pressures of the refrigerant, the heat exchangers, the compressor, the flash tank, the inlet and outlet air, and so forth. Further, other values and/or set points based on a variety of factors, such as system capacity, cooling load, and the like may be used to determine when to operate heat recovery system  35 . Controller  24  also may include components for operator interaction with the system, such as display panels and/or input/output devices for checking operating parameters, inputting set points and desired operating parameters, checking error logs and historical operations, and so forth. 
     As summarized below, controller  24  collects data, such as temperature and pressure data for the refrigerant in lines  36  and  40 , located between HRHX  34  and condenser  38  and between condenser  38  and flash tank  44 , respectively. Controller  24  may then use this data to govern operation of chiller  12 , such as the opening and closing of expansion valve  42 , which provides refrigerant to the flash tank  44 . The controller also may govern operation of chiller  12  based on other parameters, such as the temperature of water exiting HRHX  34  or the compressor capacity, which may be determined, for example, by monitoring and controlling the speed of compressor  30 . Further parameters that may be used as inputs by controller  24  for governing operation of chiller  12  may include ambient air temperature, condensing pressure, economizer operation (i.e., whether the economizer is operating and at what rate), evaporating pressure, and fan operation (i.e., whether one or more fans associated with the condenser  24  is operating and at what condition or speed). 
       FIG. 3  is a table illustrating various presently contemplated modes of operation  70  of the system of  FIG. 2 , and how certain components may be controlled in these modes. Each mode is representative of a range of heat recovery loads  72  for auxiliary heating applications and the appropriate control logic applied by the controller  24  in response to the heat recovery load  72 . The heat recovery load  72  may be a percentage of the total heat available from the refrigerant flowing through the chiller  12 . This total available heat may be equal to an amount of heat transferred from the cooling fluid to the refrigerant via the evaporator  26  added to an amount of power input to the compressor  30  for compressing the refrigerant. The heat recovery load  72  may be determined by comparing an amount of heat transferred through the HRHX  34  to this total available heat. The heat transferred from the compressed refrigerant to the process fluid via the HRHX  34  is directly related to a mass flow rate of the process fluid flowing through the HRHX  34  and a temperature difference of the process fluid between entering and exiting the HRHX  34 . In certain embodiments, the mass flow rate and temperature of the process fluid entering the HRHX  34  remain constant, such that the heat recovery load on the chiller  12  may be determined entirely based on the measured temperature of the process fluid exiting the HRHX  34 , as measured by temperature sensor  58 . As heat recovery begins, this measured temperature may be approximately equal to the temperature of the process fluid entering the HRHX  34 , such that the heat recovery load  72  is approximately 0% heat recovery. The heat recovery mode of operation  70  may be related to a temperature set point representative of a desired temperature (e.g., input by an operator) for the heated process fluid. The controller  24  may compare the measured temperature from temperature sensor  58  to the temperature set point, and when the measured temperature is below the temperature set point, the controller determines that there is a heat recovery demand. In this way, there may be a demand for heat recovery even when the heat recovery load  72  is approximately 0%. As the HRHX  34  facilitates heat transfer from the compressed refrigerant to the process fluid, the temperature of the process fluid exiting the HRHX  34  increases, thereby increasing the temperature measured by temperature sensor  58  and the determined heat recovery load  72 . Until the measured temperature reaches the temperature set point, the controller  24  controls components of the chiller  12  according to one or more of the different heat recovery modes of operation  70  described in detail below. The controller  24  is configured to determine the appropriate heat recovery mode  70  based on the measured temperature of the process fluid exiting the HRHX  34 . In addition, the controller  24  is configured to smoothly transition between the different heat recovery modes  70  as the heat recovery load  72  increases (e.g., from 0 to 100% heat recovery), until the measured temperature reaches the desired set point. 
     Each mode  70  may employ different control logic when the heat recovery load  72  falls within a given range. The different control schemes are detailed in the other columns of  FIG. 3 , which describe the hot-water flow setting  74 , the type of fan control  76 , the type of expansion valve control  78 , and the type of hot-water valve control  80  that may be employed for each of the respective modes  70 . Together, the hot-water flow setting  74 , the type of fan control  76 , the type of expansion valve control  78 , and the type of hot-water valve control  80  form the logic used by controller  24  when operating in a particular mode  70 . The hot-water flow setting  74  specifies for each mode whether the pump  60  is pumping water through the HRHX  34 . The flow rate of water from pump  60  may be controlled and monitored through another process (e.g., a different controller) that is not based on heat recovery load  72 . In certain embodiments, however, controller  24  may control the flow rate of water from pump  60  based on heat recovery load  72 . Likewise, the type of fan control  76  specifies the processing that may be used to determining an appropriate fan speed based on the desired amount of heat recovery. In addition, the type of expansion valve control  78  and the type of hot-water valve control  80  specify the type of control logic or algorithms used to determine the appropriate position of the expansion valve  42  and the three-way heat recovery valve  68 , respectively, based on the heat recovery load. 
     The controller  24  may operate in four different modes based on the desired amount of heat recovery: zero heat recovery mode  82 , low heat recovery mode  84 , intermediate heat recovery mode  86 , and full heat recovery mode  88 . Each mode  70  may be indicative of a given range of heat recovery loads (e.g., low heat recovery mode for zero to 50% heat recovery). In the zero heat recovery mode  82 , there is no heat recovery load applied to the refrigeration system, and therefore the hot-water flow from the pump  60  may be turned off, either manually or automatically by the controller  24 . 
     In zero heat recovery mode  82 , the controller operates the motor  62  at a fan speed appropriate for normal chiller operations. The term “normal chiller operations” may refer to operating the condenser fan motor  62  at a fan speed that is determined based at least in part on an ambient air temperature detected using a temperature sensor  57 . Ambient temperature may affect how the controller  24  adjusts fan operation during periods of relatively high ambient temperature. As ambient temperature increases, less heat is transferred from the condenser refrigerant to the outside air because of the reduced temperature differential. This situation may result in increased refrigerant temperature within the condenser  38 . As the temperature of the refrigerant increases, the pressure within the condenser coils may also increase. It is generally undesirable to operate the condenser coils above certain pressures. Thus, the controller  24  may automatically increase fan speed of the motor  62  in response to a high ambient temperature. The increased fan speed may facilitate additional heat transfer from the refrigerant to the outside air, thus reducing condenser pressure. In order to achieve increased chiller efficiency, normal chiller operations also may include adjusting the fan speed to reduce a combined amount of power input to the compressor  30  and power input to the fan motor  62 . The power of the compressor  30  may be calculated by the controller  24  based on a known capacity of the compressor  30  and a pressure of the refrigerant exiting the compressor, as monitored by pressure transducer  59 . 
     In the zero heat recovery mode  82 , the expansion valve may be opened by the controller  24  to a position for maintaining a desired and substantially constant subcooling of the refrigerant exiting the condenser coils  38 . The controller  24  may continually monitor the refrigerant subcooling as determined from temperature and pressure values measured by sensors  50  and  52 . This may maintain a relatively constant amount of liquid in the condenser coils  38 , which is appropriate for zero and low heat recovery requirements, but less than optimal for allowing large amounts of heat recovery from the refrigeration system. Because no hot water is pumped through the HRHX  34  when operating in the zero heat recovery mode  82 , control of the three-way heat recovery valve  68  is not employed. 
     It should be noted that the illustrated ranges of hot-water load  72  for the modes  70  are representative and may be different for different chiller designs. That is, other embodiments of chiller  12  may be designed such that the controls outlined in  FIG. 3  are desired at different ranges of heat recovery loads. For example, the ranges of hot-water load  72  for chillers  12  operating in low heat recovery mode  84  may vary (e.g., 0-30%, 0-40%, 0-60%, etc.) with the particular chiller  12 . Similarly, the ranges of hot-water load  72  for chillers  12  operating in intermediate heat recovery mode  86  may vary (e.g., 30-80%, 40-95%, 60-75%, etc.). Likewise, the ranges of hot-water load  72  for chillers  12  operating in full heat recovery mode  88  may vary (e.g., 75-100%, 80-100%, 95-100%, etc.). In other words, the low heat recovery mode may have a range of percentages between 0 and a first threshold value, and the intermediate heat recovery mode may have a range of percentages between the first threshold value and a second threshold value that is greater than the first but less than 100%. The full heat recovery modes may have a range of percentages above the second threshold value. The hot-water load  72  may therefore be divided into any appropriate ranges for applying the specified control mode  70 . 
     Low heat recovery mode  84  is the operating mode of the controller  24  when the demanded heat recovery is within a range of approximately zero to 50% heat recovery. That is, zero to 50% of the total heat to be rejected from the refrigerant between compressor system  30  and evaporator  26  is desired for an auxiliary heating function, facilitated by the HRHX  34 . In this mode, the pump  60  is operating and, therefore, the hot-water flow  74  is ON. Similar to the previous mode, the fan control  76  is based on typical chiller operations and the expansion valve control is determined based on condenser coil subcooling monitored by sensors  50  and  52 . However, unlike the previous mode of operation, low heat recovery mode  84  controls the three-way heat recovery valve  68  to bypass the HRHX  34  in order to maintain the temperature of the water supplied to the HRHX. That is, heated water exiting the HRHX  34  is sent directly to the desired heating application and not fed back toward the pump  60 . In zero or low heat recovery modes, the heat recovery bypass valve  64  may be opened to improve system performance by reducing the pressure drop of refrigerant flowing through the HRHX  34  and reducing accumulation of oil within the HRHX  34 . 
     It should be noted that both zero heat recovery mode  82  and low heat recovery mode  84  incorporate similar controls for both fan speed and expansion valve opening. Exemplary control of fan speed and expansion valve opening of such chiller systems is described in U.S. patent application Ser. No. 12/751,475, entitled “CONTROL SYSTEM FOR OPERATING CONDENSER FANS,” to Kopko et al., filed on Mar. 31, 2010; and U.S. patent application Ser. No. 12/846,959, entitled “REFRIGERANT CONTROL SYSTEM AND METHOD,” to Kopko et al., filed on Jul. 30, 2010, which are both incorporated into the present disclosure by reference. 
     The refrigeration system and controller  24  are designed to provide up to 100% heat recovery through the HRHX  34 . In full heat recovery mode  88 , the hot-water flow is indicated as ON since the pump  60  is pumping water through the HRHX  34 . Unlike the previous modes, however, the fan control is based on the temperature of hot water exiting HRHX  34 , as measured by temperature sensor  58 . When this hot water temperature increases, the controller decreases the condenser fan speed to account for the lower amount of heat to be rejected from the refrigerant in the condenser coils  38 . At 100% heat recovery, the fan(s)  63  will be turned off altogether so that the refrigerant flows through the coils without losing additional heat before entering the expansion valve  42 . In full heat recovery mode  88 , the controller  24  opens the expansion valve  42  to a position based on the subcooling of refrigerant exiting HRHX  34 , instead of the condenser coils  38 . That is, the opening of the expansion valve  42  will be selected to maintain a constant subcooling of the refrigerant from the HRHX  34 , e.g., based on a subcooling set point of approximately 5-10° F. Three-way heat recovery valve  68  is opened to allow hot water exiting the HRHX  34  to reenter the HRHX  34  until the water temperature leaving the HRHX  34 , measured by sensor  58 , reaches a threshold. This allows water to repeatedly cycle through the HRHX  34  until the desired temperature is reached, making the same HRHX structure efficient for low heat recovery applications as well as high heat recovery applications. 
     Because heat rejection through the condenser  38  is relatively low in full heat recovery mode  88 , optional coil bypass valve  66  may be opened to reduce a pressure drop of liquid refrigerant flowing through the coils of the condenser  38 . The same effect may be achieved by opening a bypass valve (not shown) around the expansion valve  42 . In this case, the bypass valve may be sized such that an appropriate flow capacity through the expansion valve  42  is realized. That is, when the expansion valve is nearly or fully opened, the bypass valve may be opened, and when the expansion valve is nearly closed, the bypass valve may be fully closed. 
     Between low and full heat recovery modes  84  and  88 , the controller  24  operates the refrigeration system in intermediate heat recovery mode  86 . For such intermediate conditions, the controls are set based on a combination of the control logic used for low heat recovery and full heat recovery. A fan speed is calculated based on the chiller controls used in low heat recovery mode  84 , another fan speed is calculated based on the hot-water temperature measured by sensor  58 , and the controller  24  drives the fan(s)  63  at the lower of the two calculated fan speeds. Similarly, positions for the expansion valve  42  are calculated based on both the subcooling of refrigerant leaving condenser coils  38  and subcooling of refrigerant leaving HRHX  34 , and the expansion valve is opened to the larger of the two openings. The three-way heat recovery valve  68  may be initially opened to allow full flow to HRHX  34  until the temperature of water exiting the HRHX reaches a threshold value, similar to the operation in full heat recovery mode  88 . In certain embodiments, if the pressure drop through condenser coils  38  is sufficiently low, the expansion valve control  78  may be based entirely on subcooling of refrigerant leaving the condenser  38 , without transitioning to different control as the heat recovery load increases. 
       FIG. 4  is a flowchart depicting an exemplary method for operating the refrigeration system. The method begins by determining (block  90 ) if the chiller system is running. If the chiller system is not running, the controller  24  may turn off (block  92 ) the condenser fan(s)  63 . If the chiller system is running, the controller  24  determines (block  94 ) if there is a demand for heat recovery from the HRHX  34  of the chiller system. The controller  24  may determine a heat recovery demand by comparing a temperature set point to a sensed temperature. For example, controller  24  may receive a signal from temperature sensor  58  indicative of the current temperature of the auxiliary water heated by HRHX  34 . Controller  24  may compare this current temperature with a temperature set point stored in controller  24  (e.g., previously input by an operator or a preset value stored in memory). If the sensed temperature is not as high as the temperature set point, a heat recovery demand exists, and the controller  24  determines the demand for heat recovery. If a heat-recovery demand does not exist, the controller  24  operates the chiller system in zero heat recovery mode  82 , as previously described. The controller may also turn off pump  60  and open heat recovery bypass valve  64 , if present, to reduce pressure drop of refrigerant through the HRHX  34 . If a heat recovery demand is detected, the controller  24  determines (block  96 ) whether the heat recovery load  72  is low. If the load is low, the controller  24  operates the fan speed, expansion valve position, and three-way hot water valve position according to the low heat recovery mode  84  as specified in  FIG. 3 . If the heat recovery demand is not low, the controller  24  determines (block  98 ) if the heat recovery load falls within the intermediate range of heat recovery values. The controller  24  then operates the chiller in intermediate heat recovery mode  86  or full heat recovery mode  88 , depending on the heat recovery load  72 . In full heat recovery mode  88 , the controller  24  may turn the fan(s) off entirely. 
       FIG. 5  is a flowchart depicting an exemplary method for operating the refrigeration system in intermediate heat recovery mode  86 . Unlike in low and full heat recovery modes, the fan speed and expansion valve position are not controlled according to readings from the same set of sensors for the full range of intermediate heat recovery loads. First the controller  24  calculates (block  100 ) a first fan speed based on chiller controls. That is, the same control logic used to determine fan speed in low heat recovery mode  84  will be used to calculate a potential fan speed in the intermediate heat recovery mode. Then, the controller calculates (block  102 ) a second fan speed based on the temperature of hot water leaving HRHX  34 , according to the same control logic used in full heat recovery mode  88 . The controller  24  drives (block  104 ) the fan motor(s)  62  at the minimum of the two calculated fan speeds. In order to also control a position of the expansion valve  42 , the controller  24  calculates a first valve opening (block  106 ) based on subcooling of the condenser coils  38  and a second valve opening (block  108 ) based on subcooling of refrigerant leaving the HRHX  34 . Then, the expansion valve  42  is opened (block  110 ) by controller  24  to a maximum of the two calculated valve openings. In this way, the expansion valve position may be controlled independently from the fan speed in the intermediate heat recovery mode  86 , allowing for relatively stable and continuous control of the refrigeration system for heat recovery loads ranging from zero to full heat recovery and across a range of ambient temperatures. 
       FIG. 6  illustrates another exemplary refrigeration system in accordance with aspects of the present technique. This system includes similar components as the refrigeration system of  FIG. 2 , but with a different configuration of the three-way heat recovery valve  68 . In this configuration, the three-way valve  68  may provide additional control of the hot water temperature output by the HRHX  34 , based on the measurements received from temperature sensor  58 , without altering condenser fan speed or expansion valve position. The three-way valve  68  may be opened so that a relatively cooler supply water is mixed with heated water exiting the HRHX  34  when the demand for heat recovery is relatively low, and the three-way valve  68  may be closed such that all supply water is pumped through the HRHX  34  to facilitate relatively higher heat recovery. In this way, the controller  24  may position the three-way heat recovery valve  68  to provide a fine adjustment of the heat recovery output temperature as the system operates in any control mode  70 . It should be noted that other arrangements and configurations of the refrigeration system may be employed, with or without certain components, e.g., optional bypass valves and the like. Additional sensors may also be used or incorporated in different configurations to provide measurements of fluid temperature within fluid lines or pressure drops across refrigeration components. Such measurements may be received by the controller  24  for monitoring and controlling operation of the refrigeration system for any desired amount of heat recovery. 
     While only certain features and embodiments of the invention have been illustrated and described, many modifications and changes may occur to those skilled in the art (e.g., variations in sizes, dimensions, structures, shapes and proportions of the various elements, values of parameters (e.g., temperatures, pressures, etc.), mounting arrangements, use of materials, orientations, etc.) without materially departing from the novel teachings and advantages of the subject matter recited in the claims. The order or sequence of any process or method steps may be varied or re-sequenced according to alternative embodiments. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention. Furthermore, in an effort to provide a concise description of the exemplary embodiments, all features of an actual implementation may not have been described (i.e., those unrelated to the presently contemplated best mode of carrying out the invention, or those unrelated to enabling the claimed invention). It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation specific decisions may be made. Such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure, without undue experimentation.