Abstract:
A heat pump includes a valve system that connects a receiver tank in fluid communication with an indoor heat exchanger, wherein the valve system, operating under a novel control scheme, works in conjunction with the receiver to control the heat pump&#39;s effective refrigerant charge. To avoid suction or discharge pressure faults and to help prevent slugs of liquid refrigerant from entering the heat pump&#39;s compressor as the heat pump switches between heating and cooling modes or switches between heating and defrost modes, the control scheme provides momentary periods of transition between those modes of operation. In some embodiments, the valve system comprises a check valve connected in parallel flow relationship with a two-position receiver valve, wherein the check valve has an appreciably higher flow coefficient than that of the receiver valve.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The subject invention generally pertains to reversible heat pump systems and more specifically to a system and method for managing the effective refrigerant charge while operating within or transitioning between various operating modes. 
         [0003]    2. Description of Related Art 
         [0004]    Compression refrigerant heat pumps with indoor and outdoor heat exchangers are operable selectively in heating and cooling modes. When the outdoor heat exchanger is air cooled (i.e., air cooled when the outdoor heat exchanger is condensing refrigerant therein), the heating mode may need to be periodically interrupted to defrost the outdoor heat exchanger by momentarily running the heat pump in a defrost mode. 
         [0005]    Shifting from one operating mode to another can cause suction or discharge pressure faults and can cause slugs of liquid refrigerant to enter the compressor, which can damage the compressor. Consequently, there is a need for addressing the problems that occur when a heat pump changes modes of operation. 
       SUMMARY OF THE INVENTION 
       [0006]    It is an object of some embodiments of the invention to provide a heat pump that avoids refrigerant pressure faults while changing from one operating mode to another. 
         [0007]    Another object of some embodiments is to provide a heat pump that helps prevent liquid slugs of refrigerant from being drawn into a compressor. 
         [0008]    Another object of some embodiments is to provide a heat pump that adjusts the effective refrigerant charge to meet the changing needs of the heat pump during heating, cooling and defrost modes of operation. 
         [0009]    Another object of some embodiments is to convey refrigerant to and from a receiver via a check valve and a two-position valve, wherein the two valves have relative flow coefficients that provide a balanced solution to somewhat conflicting concerns such as valve cost, flow rate through the receiver during the heating mode, and flow rate from the receiver during the cooling or defrost mode. 
         [0010]    In some embodiments the present invention provides a heat pump containing a refrigerant and being selectively operable in a cooling mode, a heating mode, a defrost mode, and a transition mode, wherein the transition mode occurs as the heat pump transitions from the defrost mode. The heat pump comprises a compressor connected to convey the refrigerant from a suction line to a discharge line; an outdoor heat exchanger installed between an outdoor line-A and an outdoor line-B; an indoor heat exchanger installed between an indoor line-A and an indoor line-B; a receiver; a heating expansion valve connected in fluid communication between the receiver and the outdoor line-B; a cooling expansion valve connected in fluid communication between the outdoor line-B and the indoor line-A; and a valve system connected in fluid communication between the indoor line-A and the receiver, wherein the valve system comprises a check valve and a receiver valve that are in parallel flow relationship with each other. The heat pump further comprises a directional valve connected in fluid communication with the suction line, the discharge line, the outdoor line-A, and the indoor line-B, wherein the directional valve is movable to selectively direct refrigerant flow through the heat pump. In the cooling mode and the defrost mode, the indoor heat exchanger conveys the refrigerant from the indoor line-A to the indoor line-B, the outdoor heat exchanger conveys the refrigerant from the outdoor line-A to the outdoor line-B, the cooling expansion valve is open to convey the refrigerant from the outdoor line-B to the indoor line-A, the heating expansion valve is substantially closed, and the valve system places the receiver in substantially open fluid communication with the indoor line-A. In the heating mode, the indoor heat exchanger conveys the refrigerant from the indoor line-B to the indoor line-A, the outdoor heat exchanger conveys the refrigerant from the outdoor line-B to the outdoor line-A, the cooling expansion valve is substantially closed, the heating expansion valve is open to convey the refrigerant from the receiver to the outdoor line-B, and the valve system places the receiver in substantially open fluid communication with the indoor line-A. In the transition mode, which occurs as the heat pump transitions from the defrost mode, the indoor heat exchanger conveys the refrigerant from the indoor line-A to the indoor line-B, the outdoor heat exchanger conveys the refrigerant from the outdoor line-A to the outdoor line-B, the cooling expansion valve conveys the refrigerant from the outdoor line-B to the indoor line-A, the heating expansion valve is open to allow the refrigerant to migrate from the outdoor line-B into the receiver, and the valve system is substantially closed to obstruct flow from the receiver to the indoor line-A. 
         [0011]    In some embodiments the present invention provides a method for controlling a heat pump selectively operable in a heating mode, wherein the heat pump includes an indoor heat exchanger, an outdoor heat exchanger, a compressor compressing a refrigerant, a receiver, a heating expansion valve, a cooling expansion valve, a check valve, and a receiver valve. The heating expansion valve connects the outdoor heat exchanger in fluid communication with the receiver, the cooling expansion valve connects the indoor heat exchanger in fluid communication with the outdoor heat exchanger, and the check valve and the receiver valve are connected in parallel flow relationship with each other to convey the refrigerant between the indoor heat exchanger and the receiver. The method for controlling the heat pump comprises initiating operation of the heat pump in the heating mode by running the compressor while both the check valve and the receiver valve are substantially closed; opening the check valve to convey the refrigerant at a first mass flow rate from the indoor heat exchanger to the receiver; and after opening the check valve, opening the receiver valve to convey the refrigerant at a second mass flow rate from the indoor heat exchanger to the receiver, wherein the second mass flow rate is greater than the first mass flow rate. 
         [0012]    In some embodiments the present invention provides a method for controlling a heat pump selectively operable in a heating mode, a defrost mode, and a transition mode, wherein the heat pump includes an indoor heat exchanger, an outdoor heat exchanger, a compressor compressing a refrigerant, a receiver, a heating expansion valve, a cooling expansion valve, a directional valve, a check valve, and a receiver valve. The directional valve determines a direction of flow through the indoor heat exchanger and the outdoor heat exchanger, the heating expansion valve connects the outdoor heat exchanger in fluid communication with the receiver, the cooling expansion valve connects the indoor heat exchanger in fluid communication with the outdoor heat exchanger, and the check valve and the receiver valve are connected in parallel flow relationship with each other to convey the refrigerant between the indoor heat exchanger and the receiver. The method for controlling the heat pump comprises operating the heat pump in the heating mode by releasing heat from the indoor heat exchanger and absorbing heat into the outdoor heat exchanger; and after the heating mode, operating the heat pump in the defrost mode by opening the cooling expansion valve and releasing heat from the outdoor heat exchanger; and then transitioning out of the defrost mode via a transition mode by releasing heat from the outdoor heat exchanger, closing the receiver valve to inhibit the refrigerant from flowing from the receiver to the indoor heat exchanger, and opening the heating expansion valve while the cooling expansion valve is still open. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a schematic diagram of a heat pump according to one example of the present invention with the heat pump operating in a cooling or defrost mode. 
           [0014]      FIG. 2  is a schematic diagram of the heat pump of  FIG. 1  but showing the heat pump in a heating mode. 
           [0015]      FIG. 3  is a schematic diagram similar to  FIG. 1  but showing another example heat pump. 
           [0016]      FIG. 4  is a schematic diagram of the heat pump of  FIG. 3  but showing the heat pump in a heating mode. 
           [0017]      FIG. 5  is a schematic diagram similar to  FIG. 4  but showing another example heat pump. 
           [0018]      FIG. 6  is a block diagram of a control algorithm. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0019]      FIGS. 1 and 2  show one example of a heat pump  10  that includes a receiver  12  and a valve system  14  for controlling the heat pump&#39;s effective refrigerant charge while operating within or transitioning between various operating modes. The operating modes include one or more of the following: a cooling mode for cooling a comfort zone via an indoor heat exchanger  16 , a heating mode for heating the comfort zone via indoor heat exchanger  16 , a defrost mode for defrosting an outdoor heat exchanger  18 , and one or more transition modes that occur as heat pump  10  transitions between cooling, heating and defrost modes. 
         [0020]    In some examples, the cooling mode is substantially the same as the defrost mode. The term, “heat pump” means any compression refrigerant system with at least two heat exchangers, each of which can selectively condense or evaporate the refrigerant depending on how the refrigerant is directed through the system. The terms, “indoor” and “outdoor” refer to components associated with exchanging heat (directly or indirectly) with air or some other fluid generally associated with an environment that is inside (indoor) or outside (outdoor). The terms, “indoor” and “outdoor” do not necessarily mean the related heat exchanger or component is actually physically disposed inside or outside a building. 
         [0021]    For the illustrated examples, indoor heat exchanger  16  is schematically illustrated to represent one or more heat exchangers that exchange heat with an indoor comfort zone, such as a room or other area of a building. Outdoor heat exchanger  18 , in this example, is air cooled to exchange heat with the outdoor environment. The expression, “air cooled” as used herein refers to a heat exchanger being cooled by air when the heat exchanger is operating in a mode to condense refrigerant. In addition to heat exchangers  16  and  18 , receiver  12 , and valve system  14 , heat pump  10  comprises one or more refrigerant compressors  20 , a directional valve  22 , a cooling expansion valve  24  and a heating expansion valve  26 . 
         [0022]    In the illustrated example, valve system  14  comprises a receiver valve  28  and a check valve  30 . In some examples, receiver valve  28  is a two-position solenoid actuated valve with pilot assist. Receiver valve  28  is movable selectively to an open position and a closed position. Other examples of valve  28  include, but are not limited to, a variable-open position valve, a solely fluid actuated valve, a motor actuated valve, etc. For simplicity, in some examples, valves  28  and  30  are discrete items with their own valve housings  32  and  34 , respectively. In other examples, for compactness, valves  28  and  30  are combined in a single common valve housing  36 , as shown in  FIGS. 3 and 4 . For reasons that will be explained later, receiver valve  28 , in some embodiments, has a maximum flow coefficient that is less than that of check valve  30 . 
         [0023]    Heat pump  10  also includes a controller  38  responsive to one or more inputs  46  from one or more sensors  40  for generating one or more outputs  42  that control the operation of compressor  20 , a fan system  44  (one or more fans) associated with outdoor heat exchanger  18 , directional valve  22 , expansion valves  24  and  26 , valve system  14 , and/or other components, e.g., a valve  82 , shown in  FIG. 5 . Examples of sensor  40  include, but are not limited to, a temperature sensor and a pressure sensor. In some embodiments, sensors  40  sense the suction and/or discharge pressure of compressor  20 . 
         [0024]    When operating in the cooling or defrost mode, as shown in  FIG. 1 , directional valve  22  directs relatively hot gaseous refrigerant from a compressor discharge line  48  to an outdoor line-A  50  leading to outdoor heat exchanger  18 . As refrigerant from outdoor line-A  50  passes through outdoor heat exchanger  18  to an outdoor line-B  52 , the refrigerant releases its heat to the outside air and condenses within outdoor heat exchanger  18 . When operating in the defrost mode, the heat from the condensing refrigerant is what defrosts outdoor heat exchanger  18 . 
         [0025]    During initial and normal operation in the cooling or defrost mode, heating expansion valve  26  is closed, so refrigerant from outdoor line-B  52  flows through cooling expansion valve  24 . Upon passing through cooling expansion valve  24 , the refrigerant decreases in pressure and cools by expansion. The relatively cool low pressure refrigerant enters indoor heat exchanger  16  through an indoor line-A  54 . At this point, receiver valve  28  is open, so any liquid refrigerant in receiver  12  is drawn through receiver valve  28  into the relatively low pressure indoor line-A  54 . In some embodiments of the invention, controller  38  commands valve  28  to open at a predetermined time after starting the heating mode, e.g., after about five minutes, because otherwise a high pressure differential across a closed receiver valve  28  at the beginning of the defrost mode might make it difficult for valve  28  to open. As refrigerant from indoor line-A  54  passes through indoor heat exchanger  16 , the refrigerant absorbs heat  56  from the comfort zone and vaporizes prior to exiting heat exchanger  16  through an indoor line-B  58 . Directional valve  22  directs the refrigerant from indoor line-B  58  to a suction line  60  of compressor  20 , thus perpetuating the cooling or defrost cycle. 
         [0026]    Near the end of a cooling or defrost cycle, heat pump  10  shifts to the transition mode prior to de-energizing compressor  20  or prior to initiating a heating cycle. In the transition mode, liquid refrigerant is directed to accumulate in receiver  12  to reduce the amount of liquid refrigerant in outdoor heat exchanger  18 . Otherwise, excess liquid refrigerant in outdoor heat exchanger  18  can lead to high pressure faults due to refrigerant buildup in indoor heat exchanger  16  upon restarting the heating mode. Also, upon heating mode startup, excess liquid refrigerant in outdoor heat exchanger  18  could flow into and damage compressor  20 . 
         [0027]    To change from the cooling or defrost mode to the transition mode, heating expansion valve  26  opens and receiver valve  28  closes while at least one compressor  20  continues running. This allows receiver  12  to be pressurized to just below the pressure of outdoor heat exchanger  18 . The pressure gradient between outdoor heat exchanger  18  and receiver  12  and the relatively cold wall temperature of receiver  12  encourages refrigerant to migrate from outdoor heat exchanger  18  to receiver  12  and condense there, partially filling receiver  12  with liquid refrigerant. 
         [0028]    Various trigger signals can be used for terminating the transition mode. Examples of such trigger signals include, but are not limited to, a predetermined duration of the transition mode (e.g., 45 seconds), sensor  40  sensing that the compressor&#39;s discharge pressure increased to a predetermined upper limit, and sensor  40  sensing that the compressor&#39;s suction pressure decreased to a predetermined lower limit. After terminating the transition mode, heat pump  10  can be deactivated by de-energizing compressor  20 , or heat pump  10  can be switched to operating in the heating mode. 
         [0029]    In the heating mode, shown in  FIG. 2 , heating expansion valve  26  is open, cooling expansion valve  24  is closed, and directional valve  22  directs relatively hot gaseous refrigerant from discharge line  48  to indoor line-B  58  leading to indoor heat exchanger  16 . As refrigerant from indoor line-B  58  passes through indoor heat exchanger  16  to indoor line-A  54 , the refrigerant releases its heat to the comfort zone and condenses within indoor heat exchanger  16 . 
         [0030]    Initially and until the pressure in indoor line-A  54  builds up, check valve  30  and receiver valve  28  are closed. When the pressure in indoor line-A  54  exceeds the pressure in receiver  12 , check valve  30  opens to convey refrigerant from indoor line-A  54  to receiver  12 . At a predetermined time after starting the heating mode, e.g., after about five minutes, receiver valve  28  opens to slightly reduce the flow resistance between indoor line-A  54  and receiver  12 . Thus, opening receiver valve  28  provides valve system  14  with a higher flow coefficient than when check valve  30  is open while receiver valve  28  is closed. 
         [0031]    From receiver  12 , the refrigerant flows through heating expansion valve  26 , thereby decreasing in pressure and cooling by expansion. The relatively cool low pressure refrigerant enters outdoor heat exchanger  18  through outdoor line-B  52 . As refrigerant from outdoor line-B  52  passes through outdoor heat exchanger  18 , the refrigerant absorbs heat  62  from the outside ambient air and vaporizes prior to exiting heat exchanger  18  through outdoor line-A  50 . Directional valve  22  directs the refrigerant from outdoor line-A  50  to suction line  60  of compressor  20 , thus perpetuating the heating cycle. 
         [0032]    Upon switching from the heating mode to the cooling or defrost mode, receiver valve  28  is open. So, as mentioned earlier, any liquid refrigerant that happens to be in receiver  12  is drawn through receiver valve  28  into the relatively low pressure indoor line-A  54 . To prevent receiver valve  28  from conveying an excessive inrush of liquid refrigerant from receiver  12  to indoor heat exchanger  16 , receiver valve  28  preferably provides appreciable flow resistance. In the heating mode, the flow resistance of check valve  30  should be as low as reasonably possible. To balance the various flow needs during the cooling, heating and defrost modes, the flow resistance of receiver valve  28  preferably is greater than that of open check valve  30 . 
         [0033]    In some embodiments, shown in  FIGS. 3 and 4 , a heat pump  10 ′ includes a subcooler heat exchanger  64  added to outdoor heat exchanger  18  to ensure complete condensation of refrigerant before the refrigerant enters cooling expansion valve  24  in the cooling mode. Instead of a direct line  52   a  connecting outdoor line-B  52  in fluid communication with cooling expansion valve  24 , as shown in  FIGS. 1 and 2 , subcooler  64  connects outdoor line-B  52  in fluid communication with cooling expansion valve  24 , as shown in  FIGS. 3 and 4 . Otherwise, heat pumps  10  and  10 ′ are basically the same in structure and function. 
         [0034]    In an example similar to heat pump  10 ′, shown in  FIG. 5 , a heat pump  10 ″ includes a subcooler valve  42  in addition to subcooler  64 . In this example, subcooler valve  42  is open during the cooling and defrost modes, so heat pumps  10 ′ and  10 ″ operate generally the same way during the cooling and defrost modes. During the heating mode, however, subcooler valve  42  is closed while cooling expansion valve  24  is partially open. Cooling expansion valve  24  being at least partially open allows refrigerant to migrate into subcooler  64  and accumulate there as a liquid during the heating mode. 
         [0035]    The aforementioned methods of operating and controlling heat pumps  10 ,  10 ′ and 10″ are illustrated in  FIG. 6 , which shows an example algorithm  66  under which controller  38  operates. Control block  68  illustrates operating the heat pump in the heating mode by releasing heat from the indoor heat exchanger and absorbing heat into the outdoor heat exchanger. Control block  70  illustrates: after the heating mode, operating the heat pump in the defrost mode by opening the cooling expansion valve and releasing heat from the outdoor heat exchanger. Control block  72  illustrates transitioning out of the defrost mode via a transition mode by releasing heat from the outdoor heat exchanger, closing the receiver valve to inhibit the refrigerant from flowing from the receiver to the indoor heat exchanger, and opening the heating expansion valve while the cooling expansion valve is still open. Control block  74  illustrates terminating the transition mode after either a) a predetermined duration of operation, b) if the sensed suction pressure decreases to a predetermined lower limit, or c) if the sensed discharge pressure increases to a predetermined upper limit. Control block  76  illustrates initiating operation of the heat pump in the heating mode by running the compressor while both the check valve and the receiver valve are substantially closed initially. Control block  78  illustrates: during the heating mode, opening the check valve to convey the refrigerant at a first mass flow rate from the indoor heat exchanger to the receiver. Control block  80  illustrates: after opening the check valve, opening the receiver valve to convey the refrigerant at a second mass flow rate from the indoor heat exchanger to the receiver, wherein the second mass flow rate is greater than the first mass flow rate. 
         [0036]    Although the invention is described with respect to a preferred embodiment, modifications thereto will be apparent to those of ordinary skill in the art. In some embodiments, for example, heat pumps  10 ,  10 ′ and 10″ include various service isolation valves. The scope of the invention, therefore, is to be determined by reference to the following claims: