Abstract:
To provide chilled water, a variable-primary-flow system includes two variable speed pumps that pump water through a first chiller and a second chiller. A control energizes the second chiller in response to a cooling demand exceeding that what can be met by the first chiller operating alone, and de-energizes the second chiller upon the cooling demand decreasing to a level below the first chiller&#39;s maximum capacity. When both chillers are operating, the capacities of the chillers are modulated in unison to meet the cooling demand. Likewise, when both pumps are running, their speed is modulated in unison to provide a desired pressure.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to chilled water systems that include variable flow pumps for circulating the chilled water through multiple chillers. More specifically, the present invention relates to a method of controlling such a system.  
           [0003]    2. Description of Related Art  
           [0004]    A chiller is an assembly of refrigerant components arranged in a circuit for cooling water. The chilled water is typically pumped to a number of remote heat exchangers or system coils for cooling various rooms or areas within a building.  
           [0005]    In some cases, the water may be cooled by a chiller system comprising two or more chillers. When the cooling demand is low, only one chiller of the system may need to operate, and the operating chiller&#39;s capacity may be controlled to match the demand. The cooling demand is often determined by sensing the temperature of the chilled water discharged from the chiller system and comparing the sensed temperature to a predetermined target temperature. If the cooling demand is beyond a single chiller&#39;s maximum capacity, one or more additional chillers may need to be energized. Then, the operating chillers are controlled so the system&#39;s total capacity (sum of the chillers&#39; individual capacities) meets the cooling demand.  
           [0006]    Meanwhile, the chilled water is pumped at a flow rate that is adequate for each individual chiller and is delivered at a pressure sufficient to meet the needs of the system coils. This can be accomplished by pumping the chilled water with variable speed pumps and/or controlling a bypass valve to convey a portion of the discharged chilled water back to the suction side of the pumps.  
           [0007]    Overall, controlling a chiller system can become quite involved. This is due to the difficulty of coordinating the control of several diverse chiller components, such as multiple chillers of varying capacity, multiple variable speed pumps, and a bypass valve. Moreover, the system components must operate to satisfy various needs, such as meeting the cooling demand, providing sufficient water pressure for the system coils, and providing adequate water flow through the chillers. A need to minimize the power consumption of the chillers and the chilled water pumps further complicates the controls of chiller systems. Although controls of such systems do exist, their actual control schemes may limit their use or effectiveness in certain applications, and their complexity may make them difficult to understand, install and service. Since many chiller installations have unique system requirements, there is a need for a more adaptable, straightforward control scheme for controlling chiller systems with variable speed chilled water pumps.  
         SUMMARY OF THE INVENTION  
         [0008]    It is an object of the present invention to coordinate the operation of multiple chillers, multiple variable speed pumps, and a bypass valve to meet a cooling demand.  
           [0009]    Another object of some embodiments of the invention is to energize a second chiller in response to a cooling demand exceeding that what can be met by a first chiller, and de-energizing the second chiller upon the cooling demand decreasing to a level below the first chiller&#39;s maximum capacity.  
           [0010]    Another object of some embodiments is to operate two chillers in unison, whereby the chillers operate at the same capacity with respect to a percentage of their maximum capacity.  
           [0011]    Another object of some embodiments is to operate two pumps at the same speed, but vary their speed to achieve a certain discharge pressure or pressure differential.  
           [0012]    Another object, for a chiller system having two variable speed water pumps, is to maintain sufficient water flow through two chillers by opening a bypass valve that is in parallel flow relationship with the chillers.  
           [0013]    Another object of some embodiments is to vary the speed of two pumps in response to sensing a pressure differential across a remote heat exchanger coil.  
           [0014]    One or more of these objects are provided by a chiller system that includes two variable speed pumps that pump water through a first chiller and a second chiller for cooling the water. A control energizes the second chiller in response to a cooling demand exceeding that what can be met by the first chiller operating alone, and de-energizes the second chiller upon the cooling demand decreasing to a level below the first chiller&#39;s maximum capacity.  
           [0015]    The present invention provides a method of controlling a chiller system that includes a first chiller and a second chiller through which water can be pumped to meet a cooling demand. The method comprises: pumping the water through the first chiller at a first flow rate to meet the cooling demand; increasing the cooling demand; in response to increasing the cooling demand, pumping the water through the first chiller at a second flow rate that is less than the first flow rate; and in response to increasing the cooling demand, pumping the water through the second chiller at a third flow rate, wherein the first flow rate is substantially equal to a sum of the second flow rate plus the third flow rate. The present invention also provides, with respect to the water, piping the first chiller and the second chiller in parallel flow relationship with a heat exchanger that is spaced apart from the first chiller and the second chiller, whereby the water is conveyed to the heat exchanger via a supply line and is conveyed from the heat exchanger via a return line; sensing a water pressure differential between the supply line and the return line; and controlling the first flow rate, the second flow rate and the third flow rate in response to sensing the water pressure differential.  
           [0016]    The present invention further provides a method of controlling a chiller system that includes a first chiller and a second chiller for meeting a demand for chilled water, wherein the first chiller is selectively operable at a first full load and a first range of partial loads, and the second chiller is selectively operable at a second full load and a second range of partial loads. The chiller system further includes a chilled water circuit, a first pump for forcing the chilled water through the first chiller at a first flow rate that may vary, a second pump for forcing the chilled water through the second chiller at a second flow rate that may vary, a bypass valve, a first heat exchanger, and a second heat exchanger. The chilled water circuit connects the first chiller, the second chiller, the bypass valve, the first heat exchanger, and the second heat exchanger in parallel flow relationship with respect to the flow of chilled water. The method comprises increasing the demand for chilled water; in response to increasing the demand for chilled water, changing the operation of the first chiller from operating at the first full load to operating within the first range of partial loads; in response to increasing the demand for chilled water, reducing the first rate at which the first pump forces chilled water through the first chiller; and in response to increasing the demand for chilled water, energizing the second chiller to begin operating the second chiller in the second range of partial loads. The present invention yet further provides, via a supply line of the chilled water circuit, conveying the chilled water to the first heat exchanger and the second heat exchanger; via a return line of the chilled water circuit, conveying the chilled water from the first heat exchanger and the second heat exchanger; sensing a water pressure differential between the supply line and the return line; and varying the first flow rate and the second flow rate in response to sensing the water pressure differential.  
           [0017]    The present invention still further provides a method of controlling a chiller system that includes a first chiller and a second chiller for meeting a demand for chilled water. The first chiller is selectively operable at a first full load and a first range of partial loads, and the second chiller is selectively operable at a second full load and a second range of partial loads. The chiller system further includes a chilled water circuit, a first pump for forcing the chilled water through the first chiller at a first flow rate that may vary, a second pump for forcing the chilled water through the second chiller at a second flow rate that may vary, a bypass valve, a first heat exchanger, and a second heat exchanger. The chilled water circuit connects the first chiller, the second chiller, the bypass valve, the first heat exchanger, and the second heat exchanger in parallel flow relationship with respect to the flow of chilled water. The method comprises establishing a chilled water temperature target; establishing a chilled water pressure target; selectively operating the chiller system in a high demand mode and a low demand mode to meet the chilled water temperature target; in the low demand mode, leaving the second chiller inactive while selectively operating the first chiller in the full load and the first range of partial loads to meet the chilled water temperature target; in the low demand mode, leaving the second pump inactive while modulating the pressure of the chilled water by controlling the operation of the first pump to meet the chilled water pressure target; in the high demand mode, operating the first chiller at a first partial load while operating the second chiller at a second partial load; and in the high demand mode, modulating the pressure of the chilled water by controlling the operation of the first pump and the second pump to meet the chilled water pressure target.  
           [0018]    The present invention additionally provides a method of controlling a chiller system that includes a first chiller and a second chiller for meeting a demand for chilled water. The first chiller is selectively operable at a first full load and a percent of the first full load ranging from zero to one hundred percent, and the second chiller is selectively operable at a second full load and a percent of the second full load ranging from zero to one hundred percent. The chiller system further includes a chilled water circuit, a first pump for forcing the chilled water through the first chiller at a first flow rate that may vary, a second pump for forcing the chilled water through the second chiller at a second flow rate that may vary, a bypass valve, a first heat exchanger, and a second heat exchanger. The chilled water circuit connects the first chiller, the second chiller, the bypass valve, the first heat exchanger, and the second heat exchanger in parallel flow relationship with respect to the flow of chilled water. The method comprises establishing a chilled water temperature target; establishing a chilled water pressure target; selectively operating the chiller system in a high demand mode and a low demand mode to meet the chilled water temperature target; in the low demand mode, leaving the second chiller inactive while operating the first chiller to meet the chilled water temperature target; in the low demand mode, leaving the second pump inactive while modulating the pressure of the chilled water by controlling the operation of the first pump to meet the chilled water pressure target; in the low demand mode, modulating the pressure of the chilled water by controlling the operation of the first pump and the second pump to meet the chilled water pressure target; in the high demand mode, modulating the first chiller at a percentage of the first full load; and in the high demand mode, modulating the second chiller at a percentage of the second full load and in unison with the first chiller, whereby the percentage of the first full load is substantially equal to the percentage of the second full load.  
           [0019]    The present invention moreover provides a chiller system. The system comprises a first chiller wherein the first chiller is selectively operable at a first full load and a first range of partial loads; and a second chiller for meeting a demand for chilled water wherein the second chiller is selectively operable at a second full load and a second range of partial loads. The system also comprises a first pump for forcing the chilled water through the first chiller at a first flow rate that may vary, a second pump for forcing the chilled water through the second chiller at a second flow rate that may vary; a bypass valve; a first heat exchanger; a second heat exchanger; and a chilled water circuit. The chilled water circuit connects the first chiller, the second chiller, the bypass valve, the first heat exchanger, and the second heat exchanger in parallel flow relationship with respect to the flow of chilled water; control circuitry or logic establishing a chilled water temperature target; control circuitry or logic establishing a chilled water pressure target; control circuitry or logic selectively operating the chiller system in a high demand mode and a low demand mode to meet the chilled water temperature target. The system further comprises, in the low demand mode, leaving the second chiller inactive while selectively operating the first chiller in the full load and the first range of partial loads to meet the chilled water temperature target; control circuitry or logic, in the low demand mode, leaving the second pump inactive while modulating the pressure of the chilled water by controlling the operation of the first pump to meet the chilled water pressure target; control circuitry or logic, in the high demand mode, operating the first chiller at a first partial load while operating the second chiller at a second partial load; and control circuitry or logic, in the high demand mode, modulating the pressure of the chilled water by controlling the operation of the first pump and the second pump to meet the chilled water pressure target.  
           [0020]    The present invention still further provides a chiller system. The system includes a first chiller where the first chiller is selectively operable at a first full load and a percent of the first full load ranging from zero to one hundred percent; a second chiller for meeting a demand for chilled water where the second chiller is selectively operable at a second full load and a percent of the second full load ranging from zero to one hundred percent; a first pump for forcing the chilled water through the first chiller at a first flow rate that may vary; and a second pump for forcing the chilled water through the second chiller at a second flow rate that may vary. The system also includes a bypass valve; a first heat exchanger; a second heat exchanger; and a chilled water circuit wherein the chilled water circuit connects the first chiller, the second chiller, the bypass valve, the first heat exchanger, and the second heat exchanger in parallel flow relationship with respect to the flow of chilled water. The system also includes a controller establishing a chilled water temperature target and a chilled water pressure target, the controller selectively operating the chiller system in a high demand mode and a low demand mode to meet the chilled water temperature target. In the low demand mode, the controller leaves the second chiller inactive while operating the first chiller to meet the chilled water temperature target; in the low demand mode, the controller leaves the second pump inactive while modulating the pressure of the chilled water by controlling the operation of the first pump to meet the chilled water pressure target; in the low demand mode, the controller modulates the pressure of the chilled water by controlling the operation of the first pump and the second pump to meet the chilled water pressure target; in the high demand mode, the controller modulates the first chiller at a percentage of the first full load; and in the high demand mode, the controller modulates the second chiller at a percentage of the second full load and in unison with the first chiller. The percentage of the first full load is substantially equal to the percentage of the second full load. 
       
    
    
     DESCRIPTION OF THE DRAWING FIGURES  
       [0021]    [0021]FIG. 1 is a schematic diagram of a chiller system according to one embodiment of the invention.  
         [0022]    [0022]FIG. 2 is a flow chart illustrating a control scheme for the chiller system of FIG. 1. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0023]    A chiller system  10 , shown in FIG. 1, includes multiple chillers for generating chilled water. The term, “chiller” refers to any apparatus having a refrigerant cycle for creating a cooling effect. Multiple pumps force the water through the chillers, and a chilled water circuit  12  distributes the chilled water to various system coils or heat exchangers for cooling rooms or other areas within a building. Although system  10  may include any number of chillers and pumps, for illustration, system  10  will be described as having two chillers  14  and  16 , two pumps  18  and  20 , and two coils  22  and  24 .  
         [0024]    Chillers  14  and  16  are schematically illustrated to represent all types of chillers. In one embodiment of the invention, chiller  14  includes a compressor  26  that forces a refrigerant in series through a condenser  28 , an expansion device  30  (e.g., flow restrictor, orifice, capillary, expansion valve, etc.), and an evaporator  32 . With the aid of a condenser fan  34  (or some other system for promoting the transfer of heat), condenser  28  releases waste heat from relatively hot compressed refrigerant inside condenser  28 . From condenser  28 , the refrigerant expands and its temperature drops upon passing through expansion device  30 . The cooler refrigerant then passes through evaporator  32  to cool the water that pump  18  forces through evaporator  32 . After cooling the water, the refrigerant returns to the suction side of compressor  26  to perpetuate the refrigerant cycle.  
         [0025]    Chiller  14  is preferably provided with a device that can adjust the refrigerant&#39;s flow rate for varying the chiller&#39;s capacity or cooling effect. Common examples of such a device include, but are not limited to, adjustable inlet guide vanes of a centrifugal compressor, a slide valve of a screw compressor, and a compressor driven by a variable speed motor. All of these examples and more are schematically represented by arrow  36 .  
         [0026]    In some embodiments of the invention, chillers  14  and  16  are similar in that chiller  16  includes a compressor  26 ′, a condenser  28 ′, an expansion device  30 ′ and an evaporator  32 ′. However, one chiller may have a higher maximum cooling capacity than the other.  
         [0027]    Chillers  14  and  16  may be installed in the same general location (e.g., basement or roof of the building), and system coils  22  and  24  may be installed where they are closer to the areas they cool. To connect the chillers to the coils, chilled water circuit  12  includes a supply line  38  and a return line  40 . Supply line  38  conveys chilled water from chillers  14  and  16  to coils  22  and  24 . From supply line  38 , the chilled water passes through coils  22  and  24  to cool air that a fan forces across the coils to cool the building. Valves  42  and  44  can throttle the flow of chilled water to a coil, thereby providing a way to individually control or limit the amount of cooling for a particular area of the building. After the water passes through the coils, the return line  40  conveys the water back to the inlet side of pumps  18  and  20 .  
         [0028]    To inhibit backflow through the chillers, circuit  12  may include two check valves  46  and  48 . When only one chiller/pump is operating, one of the check valves prevents the water from flowing backwards through the inactive chiller/pump. For example, if chiller  14  and pump  18  are operating while chiller  16  and pump  20  are inactive, check valve  48  prevents water in supply line  38  from flowing backwards in series through evaporator  32 ′, pump  20  and into return line  40 . Likewise, check valve  46  prevents water from flowing backwards through evaporator  32  when pump  20  is operating and pump  18  is inactive.  
         [0029]    In some situations, such as during periods of very low cooling demand, valves  42  and  44  may throttle the water flow to such an extent that the total flow rate is inadequate for chiller  14  or  16 . If the flow rate through an operating chiller becomes too low, the water might freeze inside the chiller. To avoid this, a bypass valve  50  may be partially or fully opened to create a shunt that can convey at least a portion of the water from supply line  38  directly to return line  40  without all the water having to first pass though valves  42  and  44 .  
         [0030]    To provide chilled water at a proper temperature and pressure, system  10  includes a controller  52 . Controller  52  is schematically illustrated to encompass a wide variety of electrical devices (programmable or not programmable) having the ability to provide various output signals  54  in response to various input signals  56 . Examples of controller  52  include, but are not limited to, microcomputers, personal computers, dedicated electrical circuits having analog and/or digital components, programmable logic controllers, and various combinations thereof.  
         [0031]    In some embodiments of the invention, controller  52  controls chiller system  10  according to the flow chart of FIG. 2. In decision block  58 , controller  52  compares the actual chilled water temperature to an established chilled water temperature target or set point. Controller  52  can determine the actual chilled water temperature from a temperature sensor  60  on supply line  38  and/or individual temperature sensors  62  and  64  (associated with chillers  14  and  16 , respectively). Controller  52  may receive temperature-indicating signals  66 ,  68  and  70  from temperature sensors  60 ,  62  and  64 , respectively. Establishing the chilled water temperature target can be performed through a conventional input device, such as a keyboard, dial, etc.  
         [0032]    If block  58  determines that the actual chilled water temperature is less than or equal to the set point, control decision block  70  determines whether chiller  14  should continue operating (provided it was already operating). If chiller  14  is operating below its predetermined minimum capacity, control block  72  deactivates chiller  14 , and control returns to decision block  58 . Otherwise, control shifts to control block  74 , which compares an actual chilled water pressure to an established chilled water pressure target.  
         [0033]    Establishing the chilled water pressure target can be a performed at any time before or after the installation of system  10  and may be performed through a conventional input device, such as a keyboard, dial, etc. Controller  52  can determine the actual chilled water pressure from a pressure sensor  76  (sensing pressure of water entering chiller  14 ), a pressure sensor  78  (sensing pressure of water entering chiller  16 ), a pressure sensor  80  (sensing the pressure of water leaving chiller  14 ), a pressure sensor  82  (sensing the pressure of water leaving chiller  16 ), a pressure sensor  84  (sensing the pressure of water in supply line  38 , near coil  24 ), and/or a pressure sensor  86  (sensing the pressure of water in return line  40 , near coil  24 ). The actual chilled water pressure value can be a single pressure reading or a pressure differential between two pressure readings. Controller  52  may receive pressure-indicating signals  88 ,  90 ,  92 ,  94 ,  96  and  98  from pressure sensors  76 ,  78 ,  80 ,  82 ,  84  and  86 , respectively.  
         [0034]    In a currently preferred embodiment, block  74  compares the chilled water pressure target (e.g., a delta-P value) to a pressure differential (signal  96  minus signal  98 ) across the system coil (e.g., coil  24 ) that is furthest from the chillers. In response to the comparison in block  74 , block  100  directs controller  52  to provide an output signal  102  that causes pump  18  to create a pressure differential across coil  24  that meets the target value. Controlling a pump to modulate pressure is well known to those skilled in the art. For example, pump  18  can be driven by a variable speed motor whose inverter or other control circuitry is responsive to signal  102 .  
         [0035]    In block  104 , control  52  varies the opening of bypass valve  50  via a signal  105  if the water flow through chiller  14  is too low. Controller  52  can determine the flow rate by receiving a flow rate input signal  106  from a flow sensor  108 . Alternatively, the flow rate can be determined by comparing known flow characteristics of evaporator  32  to the pressure drop across the evaporator (the difference between pressure signals  92  and  88 ).  
         [0036]    In block  110 , controller  52  provides one or more output signals  112  that vary the capacity or otherwise control chiller  14  in an attempt to meet the cooling demand with chiller  16  inactive. With only one chiller operating, system  10  is considered as operating in a low demand mode. Controller  52  generates output signal  112  in response to the chilled water temperature signal  66 , chilled water temperature signal  62 , and/or signal  114 , wherein signal  114  represents various common feedback from the operation of chiller  14 . In this example, output signal  112  represents one or more signals for varying the opening of inlet guide vanes and varying the speed of compressor  26 , thereby operating chiller  14  over a range of partial loads between zero and one hundred percent of the chiller&#39;s full load. Such control of a single chiller to meet a cooling demand can be accomplished by any of the numerous control functions well known to those skilled in the art.  
         [0037]    Periodically, decision block  116  determines whether chiller  14  is operating at its rated full load. If not, control of system  10  continues as just described. However, if chiller  14  is at full load, another decision block  118  determines whether chiller  14  is able to maintain the chilled water temperature at or below its target temperature. If chiller  14  operating at full load is sufficient to meet the cooling demand, control returns to block  58  whose function has already been defined.  
         [0038]    Referring back to block  118 , if chiller  14  is unable to meet the cooling demand, control shifts to block  120  to change the operation of system  10  to a high demand mode. In the high demand mode, block  120  directs controller  52  to provide an output signal  122  that activates pump  20 . Controller  52  now modulates both pumps  18  and  20  to create a pressure differential across coil  24  that meets the water pressure target. Upon switching from the low demand mode to the high demand mode, signal  102  will reduce the speed of pump  18 , since two pumps are now running instead of just one. Ideally, the flow rate through pump  18  operating alone during the low demand mode will be about equal to the combined flow rates through pumps  18  and  20  during the high demand mode. In the high demand mode, the speed modulation of both pumps can be simplified by controlling their speed in unison, whereby both pumps are controlled to run at the same speed or at the same percentage of their rated full speed.  
         [0039]    In block  124 , controller  52  varies the opening of bypass valve  50  if the water flow through either chiller  14  or  16  is too low. Similar to what was done with chiller  14 , controller  52  can determine the flow rate through evaporator  32 ′ by receiving a flow rate input signal  126  from a flow sensor  128 . Alternatively, the flow rate through chiller  16  can be determined by comparing known flow characteristics of evaporator  32 ′ to the pressure drop across the evaporator (the difference between pressure signals  94  and  90 ).  
         [0040]    In block  130 , controller  52  provides output signals  112  and  112 ′ to vary the capacity or otherwise control chillers  14  and  16 , respectively. With both chillers operating, system  10  is considered as operating in the high demand mode for meeting generally higher cooling demands. Controller  52  generates output signals  112  and  112 ′ in response to one or more feedback signals, such as chiller water temperature signals  66 ,  68  and  70  and/or signals  114  and  114 ′. Signals  114  and  114 ′ are similar in that they both represent various common feedbacks from the operation of their respective chiller. In this example, output signal  112 ′ represents one or more signals for varying the opening of inlet guide vanes and varying the speed of compressor  26 ′, thereby operating chiller  16  over a range of partial loads between zero and one hundred percent of the chiller&#39;s full load. In the high demand mode, the capacity of chillers  14  and  16  are preferably modulated in unison, whereby both chillers operate at the same percentage of their respective full load rating. For example, at times, both chillers operate at 50% of their full load, and other times they both chillers operate at 75% of their full load. This can be done even when one chiller has a significantly higher full load capacity than the other.  
         [0041]    Periodically, a decision block  132  determines whether system  10  can return to operating in the low demand mode. This is done by considering the combined partial loads of both chillers  14  and  16  and comparing that to the rated full load of chiller  14 . If the rated full load of chiller  14  is appreciably greater than the combined partial loads of both chillers, control block  134  will deactivate chiller  16 , and block  136  will stop pump  20 , thereby returning system  10  to its low demand mode of operation. Otherwise, control returns to block  120 , and system  10  continues operating in the high demand mode.  
         [0042]    When a chiller is operating at less than full load, the chiller&#39;s partial load can be determined in various ways that are well known to those skilled in the art. For example, the electrical current to the motor that drives the compressor can be measured (e.g., signal  114  or  114 ′), and the chiller&#39;s percent of full load can be approximated as a ratio of the motor&#39;s current draw at part load to the motor&#39;s current draw at full load. Alternatively, a chiller&#39;s load can be defined as a product of the flow rate of chilled water passing through the chiller&#39;s evaporator (e.g., signal  106  or  126 ) times the chilled water&#39;s temperature drop upon passing through the evaporator. Such a temperature drop can be determined by installing temperature sensors  150  and  152 , which provide signals  138  and  140  that indicate the temperature of the water entering evaporators  32  and  32 ′ respectively. The temperature drop will then be the value of signal  68  minus the value of signal  138  for evaporator  32 , or the value of signal  70  minus the value of signal  140  for evaporator  32 ′. Sensing the position of a compressor&#39;s inlet guide vanes, the position of a compressor&#39;s slide valve, and/or a compressor&#39;s speed are other ways of determining a chiller&#39;s operating load.  
         [0043]    Although the invention is described with reference to a preferred embodiment, it should be appreciated by those skilled in the art that other variations are well within the scope of the invention. Therefore, the scope of the invention is to be determined by reference to the claims, which follow.