Abstract:
An air conditioning system having a cooling mode and a free-cooling mode. The system having a refrigeration circuit having a compressor and a pump; a suction pressure sensor for measuring a suction pressure of the compressor; a discharge pressure sensor for measuring a discharge pressure of the compressor; a controller for selectively operating in the cooling mode by circulating and compressing a refrigerant through the refrigeration circuit via the compressor or operating in the free-cooling mode by circulating the refrigerant through the refrigeration circuit via the pump; and a recover-refrigerant sequence resident on the controller, the recover-refrigerant sequence being configured to pump the refrigerant in a portion of the refrigeration circuit not used in the free-cooling mode to remaining portions of the refrigeration circuit used in the free-cooling mode when the controller switches from the cooling mode to the free-cooling mode.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present disclosure is related to air conditioning systems. More particularly, the present disclosure is related to methods and systems for controlling air conditioning systems having a free-cooling mode and a cooling mode. 
         [0003]    2. Description of Related Art 
         [0004]    During the typical operation of air conditioning systems, the system is run in a cooling mode wherein energy is expended by operating a compressor. The compressor compresses and circulates a refrigerant to chill or condition a working fluid, such as air or other secondary loop fluid (e.g., chilled water or glycol), in a known manner. The conditioned working fluid can then be used in a refrigerator, a freezer, a building, an automobile, and other spaces with climate controlled environment. 
         [0005]    However, when the outside ambient temperature is low, there exists the possibility that the outside ambient air itself may be utilized to provide cooling to the working fluid without engaging the compressor. When the outside ambient air is used by an air conditioning system to condition the working fluid, the system is referred to as operating in a free-cooling mode. 
         [0006]    As noted above, traditionally, even when the ambient outside air temperature is low, the air conditioning system is run in the cooling mode. Running in cooling mode under such conditions provides a low efficiency means of conditioning the working fluid. In contrast, running the air conditioning system under such conditions in a free-cooling mode is more efficient. In the free-cooling mode, one or more ventilated heat exchangers and pumps are activated so that the refrigerant is circulated by the pumps and is cooled by the outside ambient air. In this manner, the refrigerant, cooled by the outside ambient air, can be used to cool the working fluid without the need for the low efficiency compressor. 
         [0007]    Accordingly, it has been determined by the present disclosure that there is a need for methods and systems that improve the efficiency of air conditioning systems having a free-cooling mode. 
       BRIEF SUMMARY OF THE INVENTION 
       [0008]    An air conditioning system having a cooling mode and a free-cooling mode. The system having a refrigeration circuit having a compressor and a pump; a suction pressure sensor for measuring a suction pressure of the compressor; a discharge pressure sensor for measuring a discharge pressure of the compressor; a controller for selectively operating in the cooling mode by circulating and compressing a refrigerant through the refrigeration circuit via the compressor or operating in the free-cooling mode by circulating the refrigerant through the refrigeration circuit via the pump; and a recover-refrigerant sequence resident on the controller, the recover-refrigerant sequence being configured to pump the refrigerant in a portion of the refrigeration circuit not used in the free-cooling mode to remaining portions of the refrigeration circuit used in the free-cooling mode when the controller switches from the cooling mode to the free-cooling mode. 
         [0009]    A method of controlling an air conditioning system having a cooling mode and a free-cooling mode is provided. The method includes switching the air conditioning system to the free-cooling mode; initiating a recover-refrigerant sequence to recover refrigerant from a portion of a refrigeration circuit that is not used during the free-cooling mode but is used during the cooling mode; and maintaining the air conditioning system in the free-cooling mode after completion of the recover-refrigerant sequence. 
         [0010]    The above-described and other features and advantages of the present disclosure will be appreciated and understood by those skilled in the art from the following detailed description, drawings, and appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0011]      FIG. 1  is an exemplary embodiment of an air conditioning system in cooling mode according to the present disclosure; 
           [0012]      FIG. 2  is an exemplary embodiment of an air conditioning system in free-cooling mode according to the present disclosure; and 
           [0013]      FIG. 3  illustrates an exemplary embodiment of a method of operating the air conditioning system of  FIGS. 1 and 2  according to the present disclosure. 
           [0014]      FIG. 4  illustrates a graph of an exemplary embodiment of the refrigerant recovery sequence according to the present disclosure. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]    Referring now to the drawings and in particular to  FIGS. 1 and 2 , an exemplary embodiment of an air conditioning system (“system”) according to the present disclosure, generally referred to by reference numeral  10 , is shown. System  10  is configured to operate in a cooling mode  12  ( FIG. 1 ) and a free-cooling mode  14  ( FIG. 2 ). 
         [0016]    System  10  includes a controller  16  for selectively switching between cooling and free-cooling modes  12 ,  14 . Advantageously, controller  16  includes a refrigerant-recovery sequence  18  (“sequence”) resident thereon that monitors pressure in system  10  during the switchover from cooling mode  12  to free-cooling mode  14 . In this manner, system  10  recovers refrigerant from system  10  components that are used in cooling mode  12 , but not in free-cooling mode  14 . This allows the pump to operate during the initiation of free-cooling mode  14  and improves pump reliability. 
         [0017]    System  10  also includes a refrigeration circuit  20  that includes a condenser  22 , a pump  24 , an expansion device  26 , an evaporator  28 , and a compressor  30 . Controller  16  is configured to selectively control either compressor  30  (when in cooling mode  12 ) or pump  24  (when in free-cooling mode  14 ) to circulate a refrigerant through system  10  in a flow direction (D). Thus, system  10 , when in cooling mode  12 , controls compressor  30  to compress and circulate the refrigerant in flow direction  30 . However, system  10 , when in free-cooling mode  14 , controls pump  24  to circulate the refrigerant in flow direction  30 . As such, free-cooling mode  14  uses less energy then cooling mode  12  since the free-cooling mode does not require the energy expended by compressor  30 . Moreover, System  10  includes a suction pressure sensor  49  and a discharge pressure sensor  51 . 
         [0018]    System  10  includes a compressor by-pass loop  32  and a pump by-pass loop  34 . System  10  includes one or more valves  36 - 1 ,  36 - 2 , and  36 - 3 . In one embodiment of the present disclosure valve  36 - 3  is a three-way valve. Valves  36  are controlled by controller  16  in a known manner. Thus, controller  16  can selectively position valves  36  to selectively open and close by-pass loops  32 ,  34  as desired. 
         [0019]    In cooling mode  12 , controller  16  controls valves  36  so that compressor by-pass loop  32  is closed and pump by-pass loop  34  is open. In this manner, system  10  is configured to allow compressor  30  to compress and circulate refrigerant in the flow direction D by flowing through pump by-pass loop  34 . 
         [0020]    In contrast, controller  16 , when in free-cooling mode  14 , controls valves  36  so that compressor by-pass loop  32  is open and pump by-pass loop  34  is closed. In this manner, system  10  is configured to allow pump  24  to circulate refrigerant in the flow direction D by flowing through compressor by-pass loop  32 . 
         [0021]    Accordingly, system  10  can condition (i.e., cool and/or dehumidify) a working fluid  38  in heat-exchange communication with evaporator  28  in both cooling and free-cooling modes  12 ,  14 . Working fluid  38  can be ambient indoor air or a secondary loop fluid such as, but not limited to, chilled water or glycol. 
         [0022]    In cooling mode  12 , system  10  operates as a standard vapor-compression air conditioning system known in the art where the compression and expansion of refrigerant via expansion device  26  are used to condition working fluid  38 . Expansion device  26  can be any known controllable expansion device such as, but not limited to a thermal expansion valve. 
         [0023]    In free-cooling mode  14 , system  10  takes advantage of the heat removing capacity of outdoor ambient air  40 , which is in heat exchange relationship with condenser  22  via one or more fans  42 , to condition working fluid  38 . 
         [0024]    Although system  10  is described herein as a conventional air conditioning (cooling) system, one skilled in the art will recognize that system  10  may also be configured as a heat pump system to provide both heating and cooling, by adding a reversing valve (not shown) so that condenser  22  (i.e., the outdoor heat exchanger) functions as an evaporator in the heating mode and evaporator  28  (i.e., the indoor heat exchanger) functions as a condenser in the heating mode. 
         [0025]    It has been determined by the present disclosure that refrigerant leaving condenser  22  can be in one of several different phases, namely a gas phase, a liquid-gas phase, or a liquid phase. When controller  16  switches system  10  to free-cooling mode  14 , pump  24  is supplied with refrigerant in the different phases until the system reaches a state of equilibrium in full circuit. 
         [0026]    After controller  16  initiates free-cooling mode  14  and during the time it takes for system  10  to reach equilibrium, pump  24  is supplied with refrigerant in the different phases. Unfortunately, when pump  24  is supplied with refrigerant in the gas or liquid-gas phases, the pump does not operate as desired. Moreover, the gas phase and/or liquid-gas phase refrigerant can cause pump  24  to cavitate, which can damage the pump and/or the pump motor (not shown). 
         [0027]    Turning off pump  24  would stop the potential damage from such cavitation, but also would result in delaying the ability for system  10  to easily switch from cooling mode  12  to free-cooling mode  14 . Advantageously, controller  16  includes sequence  18  that functions to recover refrigerant from system  10  components that are not used during free-cooling mode  14  during the time when system  10  switches out of cooling mode  12  and into free-cooling mode  14 . 
         [0028]    System  10  includes a first pressure sensor  44 , a second pressure sensor  46 , a suction pressure sensor  49 , and a discharge pressure sensor  51  in electrical communication with controller  16 . First pressure sensor  44  is positioned at an entrance  48 - 1  of pump  24 , while second pressure sensor  46  is positioned at an exit  48 - 2  of the pump. Controller  16  uses the pressures measured by first and second sensors  44 ,  46  to determine a pump pressure difference in real-time. Moreover, controller  16  operates compressor  30 , adjusts the positions of expansion device  26  and valves  36 , and monitors the pressure recorded by a third pressure sensor  49  during the switchover from cooling mode  12  to free-cooling mode  14 . 
         [0029]    The operation of sequence  18  is described in more detail with reference to  FIG. 3 .  FIG. 3  illustrates an exemplary embodiment of a method  50  of controlling system  10  having recover refrigerant in sequence  18  according to the present disclosure. 
         [0030]    Method  50 , when system  10  is operating in cooling mode  12 , includes a first free cooling determination step  54 . During first free cooling determination step  54 , method  50  determines whether the temperature of ambient air  40  is sufficient for system  10  to switch to free-cooling mode  14 . If so, method  50  then performs a free-cooling capacity check step  56  wherein system  10  is checked to determine if there is sufficient capacity to operate system  10  in free-cooling mode  14 . If so, method  50  then performs sequence  18 . 
         [0031]    Sequence  18  includes a system pump down step  60  and a low pressure equalization step  62 . Initially during sequence  18 , valve  36 - 3  is in a position in accordance with cooling mode  12 , pump  24  is off, and compressor  30  is turned off. 
         [0032]    In pump down step  60 , expansion device  26  is closed and compressor  30  is turned on. Compressor  30  remains turned on while a pressure measured by suction pressure sensor  49  is greater than a suction pressure threshold. Compressor  30  is turned off when the pressure measured by suction pressure sensor  49  is less than the suction pressure threshold. There is a pressure differential (“DP”) between suction pressure sensor  49  and discharge pressure sensor  51 . 
         [0033]    In equalization sequence  62 , compressor  30  is turned off. When DP is greater than a threshold pressure differential (“DP-threshold”), expansion device  26  is opened at a minimum rate. In one embodiment of the present disclosure, expansion device  26  is positioned approximately 10 percent of a full open position. Expansion device  26  will then close when DP is less than DP-threshold. 
         [0034]    Referring now to  FIG. 4 , a graph illustrating an exemplary embodiment of sequence  18  according to the present disclosure is shown. As can be seen, system  10  runs in free-cooling capacity check step  56  for approximately eight seconds, wherein sequence  18  is initiated. In sequence  18 , initially, valve  36 - 3  is in a position in accordance with cooling mode  12 , pump  24  is off, and compressor  30  is turned off. During pump down step  60 , expansion device  26  is closed, and compressor  30  is turned on until DP equals approximately 1500 kPa. Equalization sequence  62  is then initiated, wherein expansion device  26  is opened at a minimum while DP is greater than DP-Threshold. In the illustrated embodiment, it is seen that as DP approaches DP-Threshold, the percent opening rate of expansion device  26  decreases to a value of about 3 percent opening rate. 
         [0035]    Advantageously, it has been determined by the present disclosure that sequence  18  ensures that there is sufficient compressed refrigerant in liquid form for pump  24  to operate. This improves the reliability of pump  24  when system  10  switches into free-cooling mode  14 . 
         [0036]    After sequence  18  has been performed, method  50  switches system  10  into free cooling mode  14  at a free-cooling switching step  64 . 
         [0037]    It should be recognized that method  50  is described herein by way of example in use while system  10  is operating in cooling mode  12 . Of course, it is contemplated by the present disclosure for method  50  to find equal use when system  10  is stopped such that sequence  18  avoids pump cavitation during start-up of system  10  into free-cooling mode  14  from a stopped state. 
         [0038]    After free-cooling switching step  64 , method  50  includes a pump priming step  66 . After pump  24  has been primed by step  66 , method  50  runs in free-cooling mode  14  at step  68 . System  10  continues to run in free-cooling mode  14  until either controller  16  determines that there is a lack of system capacity at a second capacity determination step  70  or determines that pump  24  is defusing or cavitating at a pump protection step  72 . If either of these conditions are determined to be present, method  50  switches system  10  into cooling mode  12  at a cooling mode switching step  74 . 
         [0039]    It should also be noted that the terms “first”, “second”, “third”, “upper”, “lower”, and the like may be used herein to modify various elements. These modifiers do not imply a spatial, sequential, or hierarchical order to the modified elements unless specifically stated. 
         [0040]    While the present disclosure has been described with reference to one or more exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the present disclosure. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the disclosure without departing from the scope thereof. Therefore, it is intended that the present disclosure not be limited to the particular embodiment(s) disclosed as the best mode contemplated, but that the disclosure will include all embodiments falling within the scope of the appended claims.