Patent Publication Number: US-2012036878-A1

Title: Low ambient cooling kit for variable refrigerant flow heat pump

Description:
FIELD 
     The subject matter described herein relates generally to heat pumps and, more particularly, to systems and methods that facilitate low ambient temperature cooling in variable refrigerant flow heat pumps. 
     BACKGROUND 
     Heat pump systems used for air conditioning and heating commonly consist of indoor units combined with outdoor units to provide heating and cooling for indoor spaces. Cooling and heating is achieved through a vapor compression cycle. Heat is absorbed from the indoor space (cooling the space) through the indoor unit and discharged to the outdoors at the outdoor unit. Heating is achieved by reversing the cycle. Heat is absorbed from the outside at the outdoor unit and discharged to the inside through the indoor units. 
     For air-source based heat pumps, the cooling operation tends to become unstable and system capacity starts to drop off as the ambient air temperature drops below 23° F. (−5° C.). Wind blowing against the coil surfaces, which tend to be exposed to ambient conditions, can exacerbate the negative affect a drop in ambient air temperature below 23° F. (−5° C.) has on system stability and capacity. 
     Attempts to combat the negative affects of low ambient air temperature and wind have included installing wind guards that actually restrict the airflow to allow a lower outdoor operating temperature. In such designs, however, the restriction tends to be constant which negatively impacts capacity both when cooling at higher outdoor temperatures and during the heating mode. 
     It is desirable to provide systems and methods that facilitate low ambient air temperature cooling in air-source based heat pump systems. 
     SUMMARY 
     Embodiments provided herein are directed to improved systems and methods that facilitate low ambient air temperature cooling in air-source based heat pump systems. In a heat pump system used for air conditioning and heating that comprises indoor units combined with an outdoor unit to provide heating and cooling of indoor spaces, an outdoor heat pump unit preferably includes a discharge hood positionable over the fan and attachable to the top of the enclosure about the fan to capture the fan discharge. The discharge hood preferably includes a damper that is openable and closeable as a function of outside temperature. In addition, the outdoor unit preferably includes wind deflectors attachable to the sides and the back of the enclosure and positioned over the condenser coil openings in the disclosure. 
     During cooling operations below 23° F., the outdoor unit draws air around the tops and bottoms of the wind deflectors and discharges the air out the discharge hood through the damper. This air is drawn across the coil surface of the heat exchanger of the outdoor unit which is under or behind the wind deflectors. The coil surface is warmer than the outdoor air so it gives up heat to the outdoors. Since the air is below 23° F. only a small condenser circuit is active and the condenser fan is operating at its lowest speed. The condensing pressure continues to drop as the outdoor temperature continues to drop further below 23° F. To maintain a stable condensing pressure, the damper assembly partially closes to a position that is a function of the outdoor ambient temperature. By partially closing the damper assembly and reducing or constricting the opening of the discharge outlet, airflow through the condenser coil of the outdoor unit is reduced even further than enabled by the minimum speed of the fan. As the outdoor temperature continues to drop, the damper assembly continues to close further reducing the airflow. If the load in the indoor space increases, more airflow across the coil is required to still maintain a suitable condensing pressure. In this case the speed of the fan will increase to force more air through the partially closed damper. For varying load conditions, the condensing pressure is tuned or adjusted by varying the speed of the fan. 
     Other systems, methods, features and advantages of the example embodiments will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. 
    
    
     
       BRIEF DESCRIPTION OF FIGURES 
       The details of the example embodiments, including structure and operation, may be gleaned in part by study of the accompanying figures, in which like reference numerals refer to like parts. The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. Moreover, all illustrations are intended to convey concepts, where relative sizes, shapes and other detailed attributes may be illustrated schematically rather than literally or precisely. 
         FIG. 1  is a schematic of a heat pump system used for heating and cooling indoor spaces of a building. 
         FIG. 2  is a schematic showing the components of a vapor compression cycle of a heat pump system. 
         FIG. 3  is a side view of an outdoor heat pump of the heat pump system shown in  FIG. 1 . 
         FIG. 4A  is a perspective view of the outdoor heat pump sown in  FIG. 3 . 
         FIG. 4B  is a perspective view of an alternative configuration of the heat pump system shown in  FIG. 3 . 
         FIG. 5  is a side view of an outdoor heat pump with a discharge hood and wind deflectors. 
         FIG. 6  is a perspective view of the discharge hood. 
         FIG. 7  is a perspective view of an alternative configuration of the discharge hood. 
         FIG. 8  is a perspective view of a wind deflector. 
         FIG. 9  is a partial side view of a wind deflector mounted on an outdoor heat pump. 
         FIG. 10  is a side view of the outdoor heat pump illustrating operation of the outdoor heat pump. 
         FIG. 11  is a side view of the outdoor heat pump illustrating operation of the outdoor heat pump. 
         FIG. 12  is a side view of the outdoor heat pump illustrating operation of the outdoor heat pump. 
         FIG. 13  is a side view of the outdoor heat pump illustrating operation of the outdoor heat pump. 
         FIG. 14  is a schematic of the control box of the discharge hood. 
     
    
    
     It should be noted that elements of similar structures or functions are generally represented by like reference numerals for illustrative purpose throughout the figures. It should also be noted that the figures are only intended to facilitate the description of the preferred embodiments. 
     DESCRIPTION 
     Each of the additional features and teachings disclosed below can be utilized separately or in conjunction with other features and teachings to systems and methods that facilitate low ambient air temperature cooling in air-source based heat pump systems. Representative examples of the present invention, which examples utilize many of these additional features and teachings both separately and in combination, will now be described in further detail with reference to the attached drawings. This detailed description is merely intended to teach a person of skill in the art further details for practicing preferred aspects of the present teachings and is not intended to limit the scope of the invention. Therefore, combinations of features and steps disclosed in the following detail description may not be necessary to practice the invention in the broadest sense, and are instead taught merely to particularly describe representative examples of the present teachings. 
     Moreover, the various features of the representative examples and the dependent claims may be combined in ways that are not specifically and explicitly enumerated in order to provide additional useful embodiments of the present teachings. In addition, it is expressly noted that all features disclosed in the description and/or the claims are intended to be disclosed separately and independently from each other for the purpose of original disclosure, as well as for the purpose of restricting the claimed subject matter independent of the compositions of the features in the embodiments and/or the claims. It is also expressly noted that all value ranges or indications of groups of entities disclose every possible intermediate value or intermediate entity for the purpose of original disclosure, as well as for the purpose of restricting the claimed subject matter. 
     Improved systems and methods are provided that facilitate low ambient air temperature cooling in air-source based heat pump systems. Turning to the figures,  FIG. 1  shows a heat pump system  10  used for air conditioning and heating that comprises indoor units  18  and  19  combined with an outdoor unit  12  to provide heating and cooling for indoor spaces  13  of a building  11 . As shown, the outdoor unit  12  is coupled through fluid piping  16  to a fluid manifold  14 , which is in turn coupled to the various indoor units  18  and  19 . Cooling and heating is achieved through a vapor compression cycle. Heat is absorbed from the indoor space  13  (cooling the space) through the indoor units  18  and  19  and discharged to the outdoors at the outdoor unit  12 . Heating is achieved by reversing the cycle. Heat is absorbed from the outside at the outdoor unit  12  and discharged to the indoor space  13  through the indoor units  18  and  19 . 
     Alternatively, the system  10  can operate as a heat recovery system which can take heat from indoor spaces  13  using a first set of indoor units  19  for spaces requiring cooling and through the vapor compression cycle, discharge this heat to indoor spaces  13  requiring heat using a second set of indoor units  18 . This is called a simultaneous heating and cooling operation. 
     The components of a vapor compression cycle of a heat pump system  20  are depicted in  FIG. 2 . A working fluid, i.e., a refrigerant, in its gaseous state, is pressurized and circulated through the system  20  by a compressor  28 . Upon exiting the compressor  28 , the refrigerant, which is in a hot and highly pressurized vapor state, is passed through a high temperature heat exchanger  22 , commonly referred to as a condenser. The refrigerant is cooled in the heat exchanger  22  until it condenses into a high pressure, moderate temperature liquid. The condensed refrigerant then passes through an expander  24  or pressure-lowering device such as an expansion valve, capillary tube, a turbine or other work extracting device. From the expander  24 , the low pressure refrigerant then passes through another heat exchanger  26 , a low temperature heat exchanger commonly referred to as an evaporator. Through heat absorption in the evaporator  26 , the refrigerant evaporates into a vapor. Upon exiting the evaporator  26 , the refrigerant returns to the compressor  28  to repeat the cycle. 
     Referring to  FIGS. 3 and 4   a , the outdoor unit  12  or outdoor heat pump of the heat pump system  10  is shown. As depicted, the outdoor unit  12  includes a box-like enclosure  30  having a front  33 , a back  34 , sides  38  and a fan  32  mounted on the top  31  of the enclosure  30  and in communication with the interior of the enclosure  30 . The fan  32  preferably includes a variable speed outdoor fan motor. In one embodiment, the outdoor unit  32  can include a raised base  39  that extends from the bottom  37  of the enclosure  30 . In another embodiment, as depicted in  FIG. 4B , an outdoor unit  12 ′ could include two or more fans  32  and  32 ′. 
     The outdoor unit  12  further comprises an expander and a compressor coupled to a heat exchanger (see, e.g.,  FIG. 2 ). The heat exchanger preferably comprising multi-circuited condenser coils. The outdoor heat pump  12  is preferably a variable refrigerant flow heat pump wherein the compressor is preferably an inverter driven, i.e., variable speed, scroll compressor. Openings  35  formed in the back  34  and sides  38  of the enclosure  30  expose the coil surfaces  36  of the heat exchanger to outdoor ambient conditions. 
     During normal cooling operation, i.e., operation above approximately 23° F. outdoor ambient air temperature, the outdoor unit  12  draws air through the back  34  and both sides  38 , and discharges air out the top  31  of the enclosure  30 . The air is drawn across the surface  36  of the condenser coils, which is exposed on three sides, by the fan  32 . Because the coil surface  36  is warmer than the outdoor air it gives up heat to the outdoors but still maintains proper pressure of the refrigerant in the system  10  for 100% capacity operation. 
     Under varying capacity demands and varying outdoor temperatures the outdoor unit  12  has a built-in control logic that opens and closes circuits within the condenser coil and also varies the speed of the fan  32  to maintain a minimum pressure of the refrigerant in the system  10  for stable operation. The lower the temperature outside, the fewer condenser circuits are active and the lower the speed of the fan  32 . At approximately 23° F. outdoors, the minimum amount of coil circuits are active and the speed of the fan  32  is at a minimum. As the outdoor temperature continues to drop, the pressure of the refrigerant in the system also drops, which tends to cause the operation of the system to become unstable and the capacity of the system to start to drop off. Also, wind blowing against the back or side coil surfaces can make this negative impact even more dramatic since the coil surfaces are exposed. 
     In order to combat the negative impact of ambient air temperatures dropping below 23° F. and wind on the operation of the system  10 , the outdoor unit  12 , as depicted in  FIG. 5 , preferably includes a discharge hood  40  positionable over the fan  32  and attachable to the top  31  of the enclosure  30  about the fan  32  to capture the fan discharge. In addition, the outdoor unit  12  preferably includes wind deflectors  60  attachable to the sides  38  and the back  34  of the enclosure  30  positioned over the coil openings  35 . With a discharge hood  40  and wind deflectors  60  installed, the outdoor unit can maintain stable operation below 23° F., preferably down to about −13° F. and, more preferably, down to about −10° F. 
     Turning to  FIGS. 5 and 6 , the discharge hood  40  preferably includes a box like enclosure  41  with a control box  50  mounted on the exterior of the enclosure  41 . The enclosure  41  includes an opening  42  at its bottom with mounting flanges  43  disposed about the periphery of the opening  42 . The discharge hood  40  includes a discharge outlet  44  on one end or side of the enclosure  41 . The hood  40  further includes a damper assembly  45  comprising a series of damper blades  46 ,  47  and  48  that are rotatably coupled to the enclosure  41  in its interior adjacent the discharge outlet  44 . The blades  46 ,  47  and  48  extend across the outlet  44  and can be rotated to vary the opening of the discharge outlet  44 . 
     Where the outdoor unit  12 ′ comprises two fans  32  and  32 ′, as depicted in  FIG. 4B , a second hood  40 ′, as shown in  FIG. 7 , identical to the first hood  40  with the exception of the control box  50 , can be mounted to the enclosure  30  of the outdoor unit  12 ′. The first and second hoods  40  and  40 ′ are preferably controllable by the control box  50  mounted on the first or master hood  40 . The second hood  40 ′ being configured as a slave hood  40 ′ that is controllable by the master hood  40 . 
     Turning to  FIGS. 8 and 9 , the wind deflectors  60  preferably include an open box like body  61  having a face  64  and side  63  panels with mounting flanges  62  extending the length of the side panels  63 . Top and bottom plates  65  and  66  extend at an angle from the top and bottom edges of the face panel  64  to a point approximately midway along the top and bottom edges of the side panels  63 . When mounted on the enclosure  30  of the outside unit  12 , the wind deflectors  60  and the enclosure  30  form air flow openings  67  and  68  positioned at the top and bottom of the wind deflectors  60 . In addition to the wind deflectors  60  preventing wind from entering the coil face area within the enclosure  30  of the outdoor unit  12 , the special angle of the top and bottom plates  65  and  66  of the wind deflectors  60  forms an air curtain preventing excessive air from blowing up through the bottom opening  68  or down through the top opening  67  of the wind deflectors  60 . However, air can still be drawn in as needed by the inverter driven condenser fan  32 . 
     In one embodiment, a low ambient temperature kit can be provided for field installation of a discharge hood and wind deflectors. The kit preferably that includes one or more discharge hoods  40  and one or more wind deflectors  60 . Preferably, the kit would include a sufficient number of discharge hoods  40  to cover all the fans  32  of the outdoor unit  12  and a sufficient number of wind deflectors  60  to cover all coil openings in the enclosure  30  of the outdoor unit  12 . 
     Turning to  FIGS. 10 ,  11 ,  12  and  13 , operation of the outdoor unit  12  with a low ambient temperature kit installed is described. During normal operation with the outdoor air temperature above 23° F., as depicted in  FIG. 10 , the fan  32  of the outdoor unit  12  draws air around the tops and bottoms of the wind deflectors  60  and discharges air through the open damper  45  (i.e., the damper blades  46 ,  47  and  48  are horizontally aligned at an angle of 0° F. to the horizontal or top of the enclosure  31 ) and out the discharge outlet  44  of the discharge hood  40 . The air is drawn across the coil surface which is under or behind the wind deflectors  60 . The deflectors  60  are mounted on the back  34  and sides  38  of the unit  12 . Because the coil surface is warmer than the outdoor air it gives up heat to the outdoors but still maintains the proper pressure for efficient operation. Above 23° F. in cooling operation, air is not restricted at all by the discharge hood  40 . The condensing pressures are maintained by opening and closing sections in the condenser coil and also varying the speed of the fan  32 . Above 23° F. outdoor ambient temperature the operation is the same as if there were no low ambient kit installed. 
     During cooling operations below 23° F., as depicted in  FIG. 11 , the outdoor unit  12  draws air around the tops and bottoms of the wind deflectors  60  and discharges air through the damper  45  and out the discharge outlet  44  of the discharge hood  40 . The air is drawn across the coil surface of the heat exchanger which is under or behind the wind deflectors  60 . The coil surface is warmer than the outdoor air so it gives up heat to the outdoors. Since the air is below 23° F. only a small condenser circuit is active and the condenser fan  32  is operating at its lowest speed. The condensing pressure continues to drop as the outdoor temperature continues to drop further below 23° F. To maintain a stable condensing pressure, the damper assembly  45  partially closes to a position that is a function of the outdoor ambient temperature where the blades  46 ,  47  and  48  are positioned at a predetermined angle to the horizontal. By partially closing the damper assembly  45  and reducing or constricting the opening of the discharge outlet  44 , airflow through the condenser coil of the outdoor unit  12  is reduced even further than enabled by the minimum speed of the fan  32 . As the outdoor temperature continues to drop, the damper assembly  45  continues to close further reducing the airflow. If the load in the indoor space increases, more airflow across the coil is required to maintain a suitable condensing pressure. In this case the speed of the fan  32  will increase to force more air through the partially closed damper  45 . For varying load conditions, the condensing pressure is tuned or adjusted by adjusting the speed of the fan  32   
     During heating operation, as depicted in  FIG. 12 , the outdoor unit  12  coil is colder than the outdoor temperature. As air is drawn across the coil, heat is absorbed from this air. The result is the air discharged through the damper assembly  45  is colder than the incoming air. For a heat pump to operate efficiently in heating mode, the air through the coil of the outdoor unit must be unobstructed. The damper control box assembly  50  is interlocked with the outdoor unit reversing valve  70  (see  FIG. 14 ). When the unit  12  switches into heating mode (energizes the reversing valve  70 ) a relay  55  in the damper control box  50  is energized. This relay  55  breaks power to the damper actuator control circuit  50 , deenergizing the actuator motor  52 . The motor  52  has a spring return feature that drives it to the full open position. This interlocking relay function assures that the damper  45  will be wide open during the heating mode allowing full airflow and full capacity. 
     When the unit  12  is operating in heating mode, frost and/or ice will build up on the fin surfaces  36  of the heat exchanger and needs to be occasionally removed by means of a defrost cycle. As depicted in  FIG. 13 , this is accomplished by shutting off the outside fan  32  entirely and switching the reversing valve  70 . By switching the reversing valve  70 , the unit is now in essence running in the cooling mode. The coil is now being heated from the energy left in the indoor units and piping. 
     The control box  50  views this as a cooling mode and the damper  45  will partially close down to a position equal to what it would be in cooling mode for the outside ambient air temperature. For example, if it was 5° F. outside and the unit was operating in cooling, the damper may be closed 50%. Now if the unit is in defrost mode the control box  50  reacts as if it were in cooling and the damper  45  is closed to 50%. The action of the damper  45  closing during defrost and the wind deflectors  60  protecting the coil from wind will increase defrost efficiency and thus shorten the length of defrost cycle. Shorter defrost cycles will yield an overall increased heat output. 
     Turning to  FIG. 14 , the control box  50  includes a weather tight enclosure  51 , a damper actuator  52  mounted to a control board  57  within the enclosure  51 , a circuit board  53  coupled to the damper actuator  52 , a control transformer  54  coupled to the circuit board  53 , an interlock relay  55  coupled to the control transformer  54 , and a thermister  56  coupled to the circuit board  53  and extending through the enclosure  51  to the outside. The damper actuator  52  preferably comprises a motor that turns a shaft of the damper assembly  45  based on a control input. The turning of the damper shaft causes the damper  45  to either open or close. The input that causes the motor to move is preferably a control voltage signal within a control voltage signal range having upper and lower threshold voltages such as, e.g., a control voltage range of 2-10 volt DC. Between the lower and upper threshold voltages, e.g., 2 and 10 volts, is the control range. At the lower threshold voltage, the motor starts to move. At the upper threshold voltage, the motor has reached its full stroke or range of movement. The actuator  52  preferrably has a range of motion between 0° and 90°. Preferably, at a 0° angle, the damper  45  is fully open. At a 90° angle the damper  45  preferably would be fully closed. In a preferred embodiment, however, the damper  45  does not totally close. The furthest the motor will be allowed to turn the shaft of the damper  45  would be to angle position less than 90° such as, e.g., an 85° angle position, which leaves the damper  45  partially open such as, e.g., approximately 5-6% open. This limit is programmed into the circuit board  53 . 
     The relay  55  has a normally closed contact. Any time the outdoor unit  12  is in cooling mode power is allowed to flow through the contact, allowing the damper  45  to operate by closing down as the outdoor temperature drops and opening up as the outdoor temperature rises. The colder it gets, the further the damper  45  closes. 
     The main purpose of the relay  55  is to allow the damper  45  to spring return to a full open position when and if the unit  12  operates in a heating mode. The reversing valve  70  will be energized when the unit  12  goes into heating mode. The relay  55  is tied into the control board  57  at the connection for the reversing valve  70 . This energizes the coil of the relay  55 , opening the contact. When the contact opens power is disconnected from the transformer  54 , which de-energizes the control board  57  and power to the damper actuator  52 . The damper  45  then spring returns to full open position to allow full airflow through the outdoor unit  12  for full heating capacity. 
     When the unit  12  is operating in heating and a demand for defrost is required the reversing valve  70  deenergizes. This de-energizes the relay  55 . The damper  45  is now allowed to operate in cooling mode, i.e., go to a partially closed position based on the outdoor temperature. 
     The resistance of the thermistor  56  changes based on the outside temperature. The thermistor  56  protrudes through the bottom of the control box  50  so that it can sense the outdoor ambient temperature. The circuit board  53  receives a resistance value of the thermistor  56  corresponding to the outside temperature. 
     The circuit board  53  is designed to take the resistance value from the thermistor  56  and convert it to a control voltage within a control voltage range such as, e.g., 2-10 volt DC output voltage, to control the position of the damper actuator  52 . In response to the control voltage received, the damper actuator  52  rotates the damper  45  to a predetermined angle corresponding to the input voltage. At or below the lower threshold voltage, e.g., 2 volts DC, which corresponds to outside temperatures at or above an upper threshold temperature such as, e.g., 23° F., the actuator motor will retain the damper in the fully open position (i.e., at a 0° angle to the horizontal). As the DC voltage increases in accordance with a drop in outside temperature, the damper actuator  52  will rotate the damper  45  toward a closed position. Since it is preferred not to fully close the damper  45  (at a damper angle of 90°, the damper is 100% closed), the closed position of the damper  45  is preferably limited to a damper angle less than 90° such as, e.g., an angle of 85°, which corresponds to a control voltage signal input limit of less than 10 volts DC such as, e.g., 9.56 volts DC. Thus, at or above the upper threshold voltage, e.g., 9.56 volts DC, which corresponds to outside temperatures at or below a lower threshold temperature such as, e.g., 3° F., the actuator motor will retain the damper  45  in the closed position. 
     The programming of the circuit board  53  also has a built in hysteresis or differential so that the damper  45  does not move back and forth continuously based on very slight temperature fluctuations. Instead the circuit board  53  is programmed to cause the damper actuator  52  to operate or move in steps. As the outside temperature cools below a threshold temperature, the actuator  52  does not move the damper  45  toward a partially closed angle until the temperature reaches a close point temperature. As the outside temperature continues to cool, the actuator  52  does not move the damper  45  toward a further partially closed angle until the temperature reaches the next close point temperature. For example, as the outside temperature drops below 21° F., the damper  45  remains at an angle of 10° from horizontal until the temperature drops to 18° F. wherein the damper  45  closes more by rotating to an angle opening of 25° from horizontal. Similarly, as the outdoor temperature begins to warm, the actuator  52  does not move the damper  45  toward a less partially closed angle until the temperature reaches an open point temperature. For example, as the temperature warms from 3° F., the damper  45  remains at an angle of 70° from horizontal until the temperature reaches 7° F. wherein the damper  45  rotates to an angle opening of 55° from horizontal. This allows stable operation of the damper assembly  45  without a continuous hunting back and forth motion. 
     While the invention is susceptible to various modifications, and alternative forms, specific examples thereof have been shown in the drawings and are herein described in detail. It should be understood, however, that the invention is not to be limited to the particular forms or methods disclosed, but to the contrary, the invention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the appended claims.