Abstract:
A coil system for an outdoor heat exchanger in a HVAC system. The coil system comprises an integrated subcooler coil section positioned between the primary coil section and the expansion valve. A distributor combines the individual refrigerant streams from the coils of the primary coil section into a single refrigerant stream before separating the stream among the different coils of the subcooler section. The subcooler coil section is positioned such that the incoming air stream is directed directly at the subcooler coil section to maximize the difference in temperature between the air stream and the refrigerant within the subcooler section.

Description:
RELATED APPLICATION 
       [0001]    The present application claims priority to U.S. Provisional Application No. 61/655,317, filed Jun. 4, 2012, and entitled, “OUTDOOR HEAT EXCHANGER COIL”, which is hereby incorporated herein in its entirety by reference. 
     
    
     FIELD OF THE DISCLOSURE 
       [0002]    The present invention is generally directed to a heat exchanger coil system for a heat exchanger in a HVAC system. Specifically, the present invention is directed to a heat exchanger coil system for facilitating the exchange of heat between a refrigerant stream and an outdoor air stream. 
       BACKGROUND OF THE DISCLOSURE 
       [0003]    A conventional HVAC system generally comprises a compressor, an expansion valve and at least two heat exchangers. A refrigerant is sent from the compressor through the first heat exchanger to the expansion valve and sent back to the compressor after passing through the second heat exchanger. One of the heat exchangers is typically positioned to exchange heat with an interior air stream, while the other heat exchanger is positioned to exchange heat with an outdoor air stream. Depending on whether the HVAC system is functioning as a cooling system or a heating system, the exterior heat exchanger can function as either a condenser or an evaporator while the interior heat exchanger operates in the opposite capacity. 
         [0004]    The efficiency of the heat exchangers is typically gauged by determining the “temperature approach” of the heat exchanger. The temperature approach is the difference between the inlet temperature of one stream and the outlet temperature of the second stream and is commonly used as an efficiency measurement of the heat exchanger. An efficient heat exchanger has a low temperature approach while an inefficient heat exchanger has a high temperature approach. In a HVAC system, a low temperature approach occurs when the refrigerant output temperature from the heat exchanger is nearly identical to the temperature of the air stream entering the heat exchanger. 
         [0005]    A typical heat exchanger configuration for HVAC systems is a “fin and tube” configuration in which the refrigerant is fed through an elongated tube and the air stream is passed across the tube to cool or heat the refrigerant. The arrangement is effective at providing maximum contact between the refrigerant and the air stream. A challenge for exterior heat exchangers is that adverse weather conditions, such as extremely low temperatures, can cause frost to form on pipes, rob heat from the refrigerant instead of supplying heat and otherwise reduce the efficiency of the heat exchanger and HVAC system as a whole. As such, HVAC systems having exterior fin and tube heat exchangers installed in less temperate climates are often less efficient when the exterior weather conditions as less than ideal. 
         [0006]    A common feature of fin and tube heat exchangers used in HVAC systems is having a plurality of individual tube coils instead of a single elongated tube coil. The refrigerant input is typically split evenly by a distributor into amongst the coils. The refrigerant must be evenly distributed and constantly supplied to each coil to maximize the heat transfer and efficiency of the heat exchanger. However, in cooler temperatures, evenly distributing the refrigerant between the different coils is often difficult as the increased viscosity of the refrigerant and other factors cause the refrigerant to be unevenly distributed through the heat exchanger. 
         [0007]    The reduced efficiency of exterior heat exchangers in less temperate climates reduces the overall efficiency of HVAC systems installed in those climates. As energy efficiency is a primary concern with HVAC systems, the increased energy required by the HVAC system to overcome the inefficiencies caused by poor performance of the exterior heat exchanger is a significant concern. As such a need exists to improve the efficiency of exterior shell and tube heat exchangers in less temperate climates. 
       SUMMARY OF THE DISCLOSURE 
       [0008]    The present invention is directed to a coil system for an outdoor heat exchanger in a HVAC system. The coil system comprises an integrated subcooler coil section positioned between the primary coil section and the expansion valve. A distributor can be positioned between the primary coil section and the subcooler coil section to combine the individual refrigerant streams from the coils of the primary coil section into a single refrigerant stream before separating the stream among the different coils of the subcooler section. The subcooler coil section can be positioned such that the incoming air stream is directed directly at the subcooler coil section to maximize the difference in temperature between the air stream and the refrigerant within the subcooler section. 
         [0009]    In a cooling mode where the coil system acts a condenser, the condensed liquid refrigerant is drawn from the primary coil section as the refrigerant condenses and is fed into the subcooler. Continually removing the condensed liquid refrigerant improves the efficiency of the heat exchanger and overall HVAC system by dropping the condensing temperature in the primary section and lowering the amount of energy required by the compressor supplying the high pressure, gaseous refrigerant to the coil system. Similarly, positioning the subcooler such that the incoming air stream is directed at the subcooler maximizes the temperature difference between the air stream and the refrigerant stream, which facilitates further lowering of the refrigerant stream temperature to nearly the temperature of the incoming air stream. The lower refrigerant temperature leaving the coil system boosts the evaporative capacity of the refrigerant stream thereby improving the efficiency of the evaporator and the overall HVAC system. 
         [0010]    In a heating mode where the coil system acts an evaporator, the subcooler section is operated at an intermediate pressure and an intermediate temperature between the evaporator temperature and the condenser temperature. The operating conditions allow the subcooler section to still draw heat from the air stream. The smaller temperature difference between the subcooler and the air stream can also prevent the subcooler coils from frosting too quickly. This feature is particularly advantageous in wet and cold weather conditions. 
         [0011]    According to an embodiment of the present invention, the coil system can further comprise at least one Venturi distributer for distributing the fluid among coils of the primary section. Each Venturi distributer is oriented in a vertical-up or vertical-down orientation rather than at a non-vertical angle. Surprisingly, orienting the Venturi distributer in a vertical orientation avoids disruptions in the flow of refrigerant to and from the various coils of the primary coil section that can occur when the Venturi distributor is oriented in a non-vertical orientation. Similarly, orienting the Venturi distributer in a vertical orientation prevents uneven distribution of the refrigerant amongst the coils that can occur when the Venturi distributer is oriented in a non-vertical orientation. 
         [0012]    A method for exchanging heat between a liquid refrigerant and an outdoor air stream, according to the present invention, can comprise supplying a refrigerant stream to an outdoor heat exchanger having a primary coil section and subcooler coil section, each having a plurality of coils. The method can further comprise dividing the high temperature stream into a plurality of sub-streams that are each fed into a primary coil of the primary coil section. An outdoor air stream is passed across each of the primary coil sections to exchange a first quantity of heat with the subdivided refrigerant stream. The plurality of sub-streams can then recombined into a single refrigerant before being redistributed among the plurality of subcooler coils of the subcooler coil section. The outdoor air stream is passed across each of the subcooler coil sections to exchange a second quantity of heat with the subdivided refrigerant stream. The subdivided refrigerant stream is than recombined into a single refrigerant stream. 
         [0013]    The above summary of the various representative embodiments of the invention are not intended to describe each illustrated embodiment or every implementation of the invention. Rather, the embodiments are chosen and described so that others skilled in the art can appreciate and understand the principles and practices of the invention. The figures in the detailed description that follow more particularly exemplify these for embodiments. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a representative process flow diagram of a prior art HVAC system. 
           [0015]      FIG. 2  is a schematic side view of a heat exchanger coil according to an embodiment of the present invention. 
           [0016]      FIG. 3  is a partial schematic side view of a heat exchanger coil of  FIG. 1 . 
           [0017]      FIG. 4  is a perspective view of a heat exchanger coil according to an embodiment of the present invention. 
           [0018]      FIG. 5  is a side face view of a heat exchanger utilizing a heat exchanger coil according to a representative embodiment of the present invention. 
           [0019]      FIG. 6  is a partially hidden side face view of the heat exchanger of  FIG. 5  having exterior fins removed. 
           [0020]      FIG. 7  is a partial, perspective side view of the heat exchanger of  FIG. 5 . 
           [0021]      FIG. 8  is a partially hidden side view of the heat exchanger of  FIG. 5  having distributor assemblies and piping removed. 
           [0022]      FIG. 9  is a side view of the heat exchanger of  FIG. 5   
       
    
    
       [0023]    While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
       DETAILED DESCRIPTION 
       [0024]    As shown in  FIG. 1 , a HVAC system  2  for use with the present invention generally comprises a compressor  4 , a first heat exchanger  6 , an expansion valve  8  and a second heat exchanger  10 . A refrigerant stream is fed through the compressor  4  to the first heat exchanger  6 , where a first air stream is passed across the refrigerant stream to exchange a first quantity of heat with the refrigerant stream. The refrigerant stream is then fed through the expansion valve  8  and into the second heat exchanger  10  where a second air stream is passed across the refrigerant stream to exchange a second quantity of heat with the refrigerant stream. In a cooling mode for cooling the interior of the building, the first air stream is typically an outdoor air stream used to cool the refrigerant stream, while the second air stream is an indoor air stream cooled by the refrigerant stream. In a heating mode, the outdoor air stream is effectively the second air stream that supplies heat to refrigerant stream, while the indoor air stream is effectively the first air stream drawing heat from the refrigerant stream. For the purposes of this disclosure, an outdoor air stream is an air stream originating from and supplied back to the exterior of the building while an indoor air stream is an air stream from and supplied back to the interior of the building. 
         [0025]    As shown in  FIG. 2-4 , a heat exchanger  20  for exchanging a quantity of heat with an outdoor air stream, according to an embodiment of the present invention, “in cooling mode,” comprises an inlet  22 , an outlet  25 , a primary coil section  24  having a plurality of primary coils  26  and a subcooler coil section  28  having a plurality of subcooler coils  30 . The heat exchanger  20  further comprises a first header assembly  32  positioned between the inlet  22  and the primary coil section  24 , a second distributor assembly  34  positioned between the primary coil section  24  and the subcooler coil section  28 ; a third distributor assembly  36  positioned between the subcooler coil section  28  and the outlet  25 . 
         [0026]    As shown in  FIGS. 2-3 , according to an embodiment of the present invention, the plurality of primary coils  26  can be subdivided into a top section  38  and a bottom section  40 . The bottom section  40  can be positioned relative to the subcooler coil section  28  such that an air stream fed through the heat exchanger  20  will pass across the subcooler coils  30  before intersecting the primary coils  26  of the bottom section  40 . In sizing the subcooler coil section  28 , the mode of operation of heat exchanger  20  will determine the relative size of the subcooler coil section  28  relative to the primary coils section  24 . For example, in a cooling mode, subcooler coil section  28  is sized based upon pressure drop and flow rate with the desired goal of having as close an approach temperature as possible relative to the refrigerant stream and outdoor air stream. Generally, an approach temperature of at least 5° F. is desired, more preferably about 3-4° F. and even more preferably, about 2.5° F. In a heating mode, physical size of the subcooler coil section  28  can be a design factor based upon potential condensation on the exterior of the subcooler coils  30 . According to one representative embodiment of the present invention, the primary coil section  24  can comprise twenty three parallel primary coils  26  with eleven primary coils  26  in the top section  38  and twelve primary coils  26  in the bottom section  40 . In this configuration, the primary coils  26  of the top section  38  comprise 4-row coils while the primary coils  26  of the bottom section  40  comprise 3-row coils. According to one representative embodiment of the present invention, four subcooler coils  30  can make up the subcooler section  28 . 
         [0027]    In cooling operation, a refrigerant stream is fed into the inlet  22  and divided into a plurality of sub-streams by the first header assembly  32 . Each refrigerant sub-stream is fed into one of the primary coils  26  and an outdoor air stream is passed across the primary coils  26 . In a cooling configuration, the refrigerant stream is supplied from the compressor  4  as a high pressure, high temperature gaseous stream that is cooled by the outdoor air stream. As the outdoor air stream intersects the primary coils  26  a portion of each refrigerant sub-stream condenses and exits the primary coils  26  into the second distributor assembly  34  where the sub-streams are recombined into a single refrigerant stream. In a heating configuration, the refrigerant stream is supplied from the expansion valve  8  as a cooled refrigerant stream that is heated by the outdoor air stream. The heated refrigerant sub-streams are similarly recovered in the second distributor assembly  34 . In either operating mode, the recombined refrigerant stream is then separated again amongst the subcooler coils  30  for additional heat transfer. The outdoor air stream is passed across the subcooler coils  30  to either further cool the condensed refrigerant or supply additional heat in a heating configuration. In cooling, the subdivided refrigerant stream exiting the subcooler  28  is recombined into a single refrigerant stream in the third distributor assembly  36  before exiting the heat exchanger  20  through the outlet  25 . 
         [0028]    According to an embodiment of the present invention, at least one of the distributor assemblies  32 ,  34 ,  36  can comprise at least one Venturi distributer  42 . In this configuration, the Venturi distributer  42  is oriented in a vertical orientation to avoid uneven distribution of the refrigerant stream that occurs when the Venturi distributer  42  is oriented in a non-vertical orientation. 
         [0029]    Referring specifically to  FIGS. 5-9 , a heat exchanger  50  according to the present invention can comprise a frame  52  to which individual fins  53  and heat exchanger coil  54  is mounted. As seen in  FIG. 5 , fins  52  can be so closely spaced so as to provide a face side  56  with a substantially solid looking appearance. With fins  52  removed, heat exchanger coil  54  is readily visible including subcooler coil section  28  and top section  38 . 
         [0030]    While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and described in detail. It is understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.