Abstract:
A heat pump HVAC system with an integrated pressure reducer which reduces the head pressure of the system when operating in the cooling mode and thus reduces compressor workload. The heat pump HVAC system includes a compressor for compressing a refrigerant, an exterior coil positioned outside of a building, an interior coil positioned within the building, and a reversing valve for changing the flow direction of refrigerant in the refrigerant circuit. A heat exchanger is provided between the outlet of the exterior coil and the thermal expansion valve. The heat exchanger cools the refrigerant flowing between the outlet of the exterior coil and thermal expansion valve using refrigerant exiting the interior coil.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates generally to the field of heating, ventilating, and air conditioning systems. More particularly, the present invention comprises a heat pump with an integrated pressure reducer for reducing compressor workload in the cooling and heating cycles. 
         [0003]    2. Description of the Related Art 
         [0004]    Various heating, ventilating, and air conditioning (HVAC) systems are known in the prior art. Heat pumps are HVAC systems which use a circulating refrigerant as a medium to absorb and move heat from the space to be cooled to another space and subsequently dump the absorbed heat out of the system. Heat pumps typically employ a reversing valve which allows the refrigerant to be circulated in one direction for cooling applications and another direction for heating applications. 
         [0005]    A simplified schematic view of a HVAC heat pump is illustrated in  FIGS. 1 and 2 . Heat pump  10  includes compressor  12  which is supplied with a liquefied refrigerant from accumulator  14 .  FIG. 1  shows heat pump operating in a cooling state. In the cooling state, heat is collected from the inside of a house through interior coil  20  (acting as an evaporator) and rejected to the atmosphere through exterior coil  18  (acting as a condenser). Reversing valve  16  directs a stream of hot compressed gas to exterior coil  18  where heat is transferred to an outdoor heat sink. Although not shown in this illustration, a fan is typically used to increase convective heat transfer via exterior coil  18 . As heat is rejected to the heat sink (atmosphere) in exterior coil  18 , the hot compressed gas turns into a hot condensed liquid. The hot condensed liquid stream passes through bypass valve  24  in the direction of interior coil  20 . At the entrance of interior coil  20 , the hot condensed liquid passes through thermal expansion valve  26  where the stream expands into a cooled vapor stream. The cooled vapor stream passes through interior coil  20  and collects indoor heat. A receiver or dryer is typically used to collect condensed moisture, but has been omitted in the view. The cooled vapor stream eventually passes through reversing valve  16  and back to accumulator  14 . 
         [0006]      FIG. 2  illustrates heat pump  10  operating in the heating mode. In the heating mode, reversing valve  16  directs a stream of hot compressed vapor from compressor  12  to interior coil  20  (which is acting as a condenser). Heat is released to the inside of the house when the hot compressed vapor stream passes through interior coil. A fan is customarily used to facilitate heat transfer via interior coil  20 . As heat is released through interior coil  20  the compressed vapor stream turns to a liquid state. The liquefied refrigerant stream passes through bypass valve  28  in the direction of exterior coil  18 . The liquefied refrigerant stream then passes through thermal expansion valve  22  where the refrigerant becomes a vapor and absorbs heat from the outside passing through exterior coil  18  (which is acting as evaporator). The vapor refrigerant is then directed back through reversing valve  16  to accumulator  14 . 
         [0007]    The heating mode performance of HVAC systems are typically evaluated in terms of coefficients of performance (COP), and cooling mode performance is evaluated in terms of energy efficiency ratio (EER) or seasonal energy efficiency ratio (SEER). EER is essentially the ratio of cooling capacity in Btu/Hr and the input power in watts (W) at a given operating point. SEER is related to EER. While EER is evaluated with respect to a specific internal and external temperature, the SEER is determined over a range of expected external temperatures (the normal temperature distribution for the geographical location of the SEER test). 
         [0008]    The amount of input power required to operate a heat pump is principally dictated by the workload and efficiency of the compressor. In the cooling mode, the compressor must generate a sufficient pressure differential to drive a hot compressed vapor stream through a thermal expansion valve. When cooling demands are elevated, the compressor requires even more input power. 
         [0009]    Because energy costs for driving HVAC systems are so substantial, measures which improve a systems energy efficiency ratio and/or reduce the compressors workload are needed. 
       BRIEF SUMMARY OF THE PRESENT INVENTION 
       [0010]    The present invention generally comprises a heat pump HVAC system with an integrated pressure reducer which reduces the head pressure of the system when operating in the cooling mode and thus reduces compressor workload. The heat pump HVAC system includes a compressor for compressing a refrigerant, an exterior coil positioned to exchange heat with the environment outside the building, an interior coil positioned to exchange heat with the interior of the building, and a reversing valve for changing the flow direction of refrigerant in the refrigerant circuit. A heat exchanger is provided between the outlet of the exterior coil and the thermal expansion valve. The heat exchanger cools the refrigerant flowing between the outlet of the exterior coil and thermal expansion valve using refrigerant exiting the interior coil. 
         [0011]    The heat pump HVAC system of the present invention is able to attain a higher energy efficiency ratio (EER) and seasonal energy efficiency ratio (SEER) than an identical system which does not employ the pressure reducer. These performance gains are largely realized by the reduced head pressure of the system caused by cooling the refrigerant before it passes through the thermal expansion valve. The heat pump HVAC system of the present invention is able to achieve this reduced head pressure without significantly affecting the system&#39;s ability to move heat. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0012]      FIG. 1  is a schematic, illustrating a prior art heat pump operating in cooling mode. 
           [0013]      FIG. 2  is a schematic, illustrating a prior art heat pump operating in heating mode. 
           [0014]      FIG. 3  is a schematic, illustrating operation of the present invention in cooling mode. 
           [0015]      FIG. 4  is a schematic, illustrating operation of the present invention in heating mode. 
       
    
    
     REFERENCE NUMERALS IN THE DRAWINGS 
       [0016]      
         [0000]    
       
         
               
               
               
               
             
           
               
                   
               
             
             
               
                 10 
                 heat pump 
                 12 
                 compressor 
               
               
                 14 
                 accumulator 
                 16 
                 reversing valve 
               
               
                 18 
                 exterior coil 
                 20 
                 interior coil 
               
               
                 22 
                 thermal expansion valve 
                 24 
                 bypass valve 
               
               
                 26 
                 thermal expansion valve 
                 28 
                 bypass valve 
               
               
                 30 
                 heat exchanger 
                 32 
                 dryer filter 
               
               
                 40 
                 heat pump 
                 42 
                 first port 
               
               
                 44 
                 second port 
                 46 
                 third port 
               
               
                 48 
                 fourth port 
               
               
                   
               
             
          
         
       
     
       DETAILED DESCRIPTION OF THE INVENTION 
       [0017]    The present invention, heat pump  40 , is illustrated in  FIGS. 3 and 4 .  FIG. 3  illustrates the operation of heat pump  40  in cooling mode and  FIG. 4  illustrates the operation of heat pump  40  in heating mode. Reversing valve  16  may be selectively positioned in a heating position ( FIG. 3 ) or a cooling position ( FIG. 4 ) to control the direction a refrigerant flows through the heat pump circuit. 
         [0018]    Turning to  FIG. 3 , heat pump  40  is illustrated in the cooling mode. In the cooling mode, interior coil  20  acts as an evaporator and exterior coil  18  acts as a condenser. Reversing valve  16 , positioned in the cooling position, directs refrigerant flow from compressor  12  to exterior coil  18 . Exterior coil  18  is positioned outside of the building cooled by heat pump  40  and transmits heat from the refrigerant flowing through exterior coil  18  to a heat sink (such as the surrounding atmosphere). As heat is transmitted via exterior coil  18 , the refrigerant liquefies. In the cooling mode, bypass valve  24  is opened to direct refrigerant flow around thermal expansion valve  22 . 
         [0019]    From bypass valve  24 , the refrigerant flows to heat exchanger  30 . Heat exchanger  30  acts as a counter-flow heat exchanger in which cooled refrigerant exiting interior coil  20  flows over a conductive conduit which transports the hot stream of refrigerant from exterior coil  18  to thermal expansion valve  26 . Heat is transferred from the hot stream to the cool stream in heat exchanger  30 . 
         [0020]    The hot stream then passes through dryer filter  32  and evaporates to a cooled gas through thermal expansion valve  26 . Those that are skilled in the art know that the cooling of the gas is caused by the reduction in pressure of the gas as it passes through the expansion valve. The ideal gas law provides that the state of an amount of gas is determined by its pressure, temperature, and volume according to the equation: 
         [0000]      PV=nRT 
         [0000]    where P is absolute pressure, V is volume occupied by the gas, n is the amount of substance of gas (expressed in moles), R is the ideal gas constant and T is absolute temperature. In accordance with this relationship, reducing the pressure of a gas results in a corresponding reduction in temperature of the gas. 
         [0021]    The cooled refrigerant vapor passes through interior coil  20  where heat from the interior of the building is transferred to the refrigerant passing through interior coil  20 . As mentioned previously, this refrigerant passes through heat exchanger  30  where it is used to cool the hot stream of refrigerant. From heat exchanger  30  the refrigerant passes back through reversing valve  16  before collecting in accumulator  14 . 
         [0022]    Turning to  FIG. 4 , heat pump  40  is illustrated in the heating mode. In the heating mode, interior coil  20  acts as a condenser and exterior coil  18  acts as an evaporator. 
         [0023]    Reversing valve  16 , positioned in the heating position, directs hot compressed refrigerant vapor from compressor  12  to interior coil  20 . Interior coil  18  transmits heat from the refrigerant flowing through interior coil  20  to the interior of the building. As heat is transmitted via interior coil  18 , the refrigerant liquefies. In the heating mode, bypass valve  28  is opened to direct refrigerant flow around thermal expansion valve  26 . 
         [0024]    From bypass valve  28 , the refrigerant flows through dryer filter  32  to heat exchanger  30 . In the heating mode heat exchanger  30  acts as a parallel-flow heat exchanger in which cooled refrigerant exiting exterior coil  18  flows over a conductive conduit which transports the hot stream of refrigerant from interior coil  20  to thermal expansion valve  22 . Heat is transferred from the hot stream to the cool stream in heat exchanger  30 . 
         [0025]    The hot stream then evaporates to a cooled gas through thermal expansion valve  22 . The cooled refrigerant vapor passes through exterior coil  18  where heat from the outdoor air is transferred to the refrigerant passing through exterior coil  18 . As mentioned previously, this refrigerant passes through heat exchanger  30  where it is used to cool the hot stream of refrigerant. From heat exchanger  30  the refrigerant passes back through reversing valve  16  before collecting in accumulator  14 . 
         [0026]    With the operation of the present invention now explained, the many advantages offered by the present invention may now be apparent to one that is skilled in the art. The reader will note that in both operating modes, heat exchanger  30  cools the “hot” stream of refrigerant before it passes through the thermal expansion valve. On a hot day, where ambient temperatures are approximately 100 degrees Fahrenheit, heat exchanger  30  may reduce the temperature of refrigerant flowing through thermal expansion valve  26  from 100 degrees Fahrenheit (in a conventional system operating without heat exchanger  30 ) to 40 degrees Fahrenheit (the temperature of refrigerant fourth port  48  of heat exchanger  30 ). This reduction in temperature (60 degrees Fahrenheit in preceding example) dramatically reduces the peak head pressure of heat pump  10  and the workload of compressor  12 . The heat pump HVAC system of the present invention is able to achieve this reduced head pressure without significantly affecting the system&#39;s ability to move heat. Thus, by adding heat exchanger  30  to an existing heat pump system, a user is able to attain a higher energy efficiency ratio (EER) and seasonal energy efficiency ratio (SEER). 
         [0027]    Such a reduction in temperature and head pressure has been observed in multiple field tests. In these field tests, a reduced compressor “amperage draw” was also observed. In many cases, the amperage draw was reduced by as much as fifty (50) percent. As such, it is estimated that he addition of such a heat exchanger in the heat pump circuit as shown in  FIG. 3  and  FIG. 4  can approximately double the SEER rating of a HVAC system. 
         [0028]    In addition, the proposed configuration of the preferred embodiment allows heat exchanger  30  to act as a counter-flow heat exchanger only during cooling mode. The reader will note that whether in heating or cooling mode, refrigerant always flows from third port  46  to first port  42 . In cooling mode, refrigerant flows from second port  44  to fourth port  48 ; however, in heating mode, refrigerant flows from fourth port  48  to second port  44 . This allows the AT (temperature differential measured from inlet to outlet) of the hot refrigerant stream passing through heat exchanger  30  to be maximized in the cooling mode where reducing the workload of compressor  12  is most beneficial. 
         [0029]    Those that are skilled in the art will realize that the present invention may be easily retrofitted to existing heat pump systems without requiring the addition or replacement of expensive components (such as compressor  12 , interior coil  20 , or exterior coil  18 ). Further, heat exchanger  30  may be easily plumbed to the existing refrigerant circuit in minimal time. Such a retrofit has been performed in field tests. In one field test, a heat exchanger was added (as shown in  FIGS. 3 and 4 ) to a 2.5 ton 13 SEER heat pump HVAC system. No components of the system were changed apart from the addition of the heat exchanger and the conduits and couplings needed to plumb the heat exchanger to the system. The system originally had a compressor amperage draw of 14.6 amps before the heat exchanger was added. After the heat exchanger was added, the amperage draw was measured to be 6.5 amps with a head pressure of 125 psi. This reduction in amperage draw boosts the efficiency rating of the system from 13 SEER to more than 26 SEER. 
         [0030]    In these retrofit field tests it was further observed that the amount of liquid refrigerant passing through accumulator  14  into compressor  12  was substantially reduced when heat exchanger  30  was added to the heat pump circuit. Those that are skilled in the art know that an electric heater is often used to preheat refrigerant before the refrigerant enters the compressor since the presence of liquid refrigerant in the compressor can damage the compressor. Such a component is not needed in the proposed heat exchanger circuit because the refrigerant is heated in heat exchanger  30  before being transmitted to accumulator  14 . The removal of this electric heater would further reduce the total amperage draw of the HVAC system. 
         [0031]    Although the preceding descriptions contain significant detail they should not be viewed as limiting the invention but rather as providing examples of the preferred embodiments of the invention. Accordingly, the scope of the invention should be determined by the following claims, rather than the examples given.