Patent Publication Number: US-2006005571-A1

Title: Refrigerant system with reheat function provided by auxiliary heat exchanger

Description:
BACKGROUND OF THE INVENTION  
      This application relates to the provision of a reheat function in a refrigerant system wherein an auxiliary heat exchanger is utilized to subcool refrigerant approaching an evaporator.  
      Refrigerant systems are utilized to control the temperature and humidity of air in various environments. In a typical refrigerant system, a refrigerant is compressed in a compressor and delivered to a condenser. In the condenser, heat is exchanged between outside ambient air and the refrigerant. From the condenser, the refrigerant passes to an expansion device, at which the refrigerant is expanded to a lower pressure and temperature, and then to an evaporator. In the evaporator heat is exchanged between the refrigerant and the indoor air, to condition the indoor air. When the refrigerant system is operating, the evaporator cools the air that is being supplied to the indoor environment. In addition, as the temperature of the indoor air is lowered, moisture usually is also taken out of the air. In this manner, the humidity level of the indoor air can also be controlled.  
      In some cases, the air, that is brought to provide a comfort environment in a conditioned space, may need to be additionally dehumidified to provide the desired humidity level. This has presented design challenges to refrigerant cycle designers. One way to address such challenges is to utilize various schematics incorporating reheat coils. In many cases, the reheat coils, placed on the way of indoor air stream are employed for the purpose of reheating at least a portion of the air supplied to the conditioned space after it has been overcooled in the evaporator, where the moisture has been removed.  
      One of the options available to a refrigerant system designer to increase efficiency and capacity of the refrigerant system is to employ an auxiliary economizer circuit. In the economizer circuit, a portion of the refrigerant flowing from the condenser is tapped and passed through an economizer expansion device and then to an economizer heat exchanger. This tapped refrigerant subcools a main refrigerant flow that also passes through the economizer heat exchanger. The tapped refrigerant leaves the economizer heat exchanger, usually in a vapor state, and is injected back into the compressor at an intermediate compression point (or in case of two compressors connected in series, the injection is taken place into the point between the low and high pressure compressors). The main portion of refrigerant flow leaving the condenser is additionally subcooled after passing through the economizer heat exchanger. The main portion of the refrigerant flow then passes through a main expansion device and then to an evaporator. This main flow will have a higher cooling capacity because it was additionally subcooled in the economizer heat exchanger. An addition of economizer circuit thus provides enhanced system performance. In an alternate arrangement, a portion of the refrigerant is tapped and passed through the economizer expansion device after being passed through the economizer heat exchanger (along with the main flow). In all other aspects this arrangement is identical to the configuration described above.  
      Another option to increase the system capacity and efficiency is to utilize a liquid-suction heat exchanger. The purpose of the liquid-suction heat exchanger is to boost system performance by providing extra subcooling to liquid refrigerant exiting the condenser coil through the heat transfer interaction with the refrigerant vapor exiting the evaporator. Although during this heat transfer interaction the refrigerant density and subsequently mass flow rate at the compressor suction are undesirably reduced, in general, the combined effect of the liquid-suction heat exchanger addition allows for overall system performance enhancement.  
      Recently, the assignee of this application has developed a system that combines the reheat coil and economizer cycle. However, variations of this basic concept have yet to be fully developed.  
     SUMMARY OF THE INVENTION  
      In disclosed embodiments of this invention, an auxiliary heat exchanger is provided and placed on the way of predominantly liquid refrigerant flow downstream of the condenser and upstream of the evaporator, and this main flow of refrigerant in the auxiliary heat exchanger is further subcooled by another refrigerant stream. In the case of the economizer heat exchanger, the latter, smaller portion of the refrigerant is evaporated and returned into the intermediate compression stage. In case of liquid-suction heat exchanger, the refrigerant in the main suction line, that is predominantly in a vapor state, before flowing into the compressor suction port, subcools the refrigerant in the main liquid line, as it leaves the condenser. In either case, the refrigerant approaching the evaporator is subcooled such that a greater cooling capacity is provided. With these arrangements, the refrigerant flow returning to the compressor through the intermediate port, in the case of the economizer heat exchanger, or through the suction port, in case of the liquid-suction heat exchanger, is being preheated by the opposite stream that is being subcooled. The present invention utilizes this preheating effect to provide a reheat function.  
      More specifically in the embodiment dealing with the liquid-suction heat exchanger, refrigerant downstream of the evaporator is utilized to subcool the refrigerant approaching the evaporator. In this case, a portion of the suction line leading to the compressor inlet or liquid suction heat exchanger itself is placed in the path of at least a portion of an airflow passing over the evaporator. As described above, the reheat function is operable to heat the air to a desired temperature, while providing the air dehumidification in the evaporator. As the air is heated while passing over the liquid-suction heat exchanger, it cools the refrigerant in this liquid-suction heat exchanger, which is desirable, as it improves the system capacity and efficiency.  
      In other embodiments, the subcooling function is provided by an economizer circuit. As is known, an economizer circuit includes a heat exchanger receiving a main flow of refrigerant and a smaller, tapped flow of refrigerant. The tapped flow of refrigerant subcools the main flow of refrigerant. The tapped flow of refrigerant is returned to an intermediate point in the compression cycle of the compressor. Since the smaller, or tapped flow, has subcooled the main flow of refrigerant, this tapped refrigerant is being evaporated or pre-heated, as it passes through the economizer heat exchanger. Thus, if the economizer heat exchanger or its return line to the compressor are placed in the heat transfer relationship with the airflow passing over the evaporator, then the refrigerant in this line is cooled by this airflow. This additional cooling of refrigerant vapor, as it enters the intermediate injection port, is desirable, as it improves system capacity and efficiency. At the same time, the reheating of the airflow is accomplished and dehumidification function is provided.  
      These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  shows a first schematic of a refrigerant system.  
       FIG. 2  shows a second schematic of a refrigerant system.  
       FIG. 3  shows a third schematic of a refrigerant system.  
       FIG. 4  shows a fourth schematic of a refrigerant system.  
       FIG. 5  shows an exemplary heat exchanger.  
       FIG. 6  shows a second embodiment heat exchanger. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
      A refrigerant system  20  is illustrated in  FIG. 1  having a compressor  22  delivering a refrigerant to a condenser  24 . An auxiliary heat exchanger  26  (liquid-suction heat exchanger in this case) is positioned downstream of the condenser  24 , and upstream of a main expansion device  28 . The refrigerant passes through an evaporator  30  downstream of the main expansion device  28 , and an air moving device  32  moves air over the evaporator  30 , with the air then being moved into an environment to be conditioned. As can be seen in  FIG. 1 , downstream of the evaporator  30 , the refrigerant once again enters an auxiliary liquid-suction heat exchanger  26  at point  34 , and exits at point  36 , subcooling the refrigerant passing to the main expansion device  28 . Thus, when the refrigerant reaches the evaporator  30 , it will have a greater cooling capacity than it otherwise would possess. As can be appreciated, the refrigerant passing from the condenser  24  to the main expansion device  28  is generally in a liquid state, while the refrigerant passing from point  34  to  36  is generally a vapor.  
      As shown in  FIG. 5 , the heat exchanger  80 , receives the liquid refrigerant passing to the main expansion device  28  through a central tube  82  (or multiple tubes), and receives the vapor refrigerant through an inlet port  84 . The vapor refrigerant then enters the space confined between a heat exchanger shell  86  and the central tube  82 . After heat transfer interaction with the liquid refrigerant inside the tube, the heated vapor refrigerant exits through an outlet port  88 . Thus, the two flows of refrigerant are maintained separated within the heat exchanger  80  and communicate with each other through the heat transfer interaction only. It is known to a person ordinarily skilled in the art that, the heat exchanger can have enhanced heat transfer surfaces (such as fins, ribs, etc) located on the outside or inside portion of the heat exchanger shell  86  in order to improve the heat transfer between the two refrigerant flows and surrounding air. The internal and external surfaces of the tube  82  can also be similarly enhanced to boost the heat transfer between the two refrigerant streams. In the heat exchanger exhibited in  FIG. 5 , the refrigerant vapor, being on the shell side, is in direct heat transfer interaction with surrounding air. Another heat exchanger configuration, with the refrigerant liquid being on the shell side, is also possible, and selection of a particular design depends on the application requirements and specifics. Thus, the connections can simply be reversed such that the liquid refrigerant is on the shell side. Finally, as shown in  FIG. 6 , in an embodiment  200 , both the vapor and the liquid are exposed to the outside environment or air, with a vapor flow path  202 , and a liquid flow path  204 . Obviously, the heat exchanger  200  can have various flow patterns and configurations, including multiple channels on both liquid and vapor sides. It should be noted that the heat exchanger designs shown in  FIGS. 5 and 6  are exemplary and many other concepts and configuration variations are also permissible to obtain full benefits of the invention teachings.  
      Downstream of the point  36 , the refrigerant passes into a suction line  41  leading back to a line  44  to direct the refrigerant back to the compressor  22 . An enhanced heat transfer surface  42  is preferably placed on or built into the suction line  44 . This heat transfer surface and the portion of the suction line are placed in the airflow having passed over the evaporator  30 . A worker of ordinary skill in the art would recognize how and when to control flow control devices, such valves  40  and  48 , to either bypass the heat exchange portion  42 , or pass the refrigerant through the heat exchange portion  42  when a reheat function is desired. It should also be noted that valves  40  and  48  and line  46  can be removed altogether if the reheat option is to remain operational all the time. The valves  48  and  42  can also be combined into a single three-way valve. Also, the valves  40  and  48  could be fixed or regulating flow control devices.  
      While the air reheating function is provided, the temperature of the refrigerant in the heat exchange portion  42  will be reduced as air temperature is expected to be colder than refrigerant temperature in the heat exchange portion  42 . This reduction in the refrigerant temperature is desirable, as it will improve capacity and efficiency of the refrigerant system by increasing the density and consequently the mass flow rate of the refrigerant entering the compressor  22 .  
       FIG. 2  shows a refrigerant system  50  that is similar to the  FIG. 1  embodiment, however, the auxiliary liquid-suction heat exchanger  126  is itself placed in the path of the airflow provided by the air moving device  32 . The temperature of the refrigerant vapor is reduced by the airflow, which is beneficial to improving the system performance, as it reduces the temperature of the refrigerant entering the compressor, as well as provides additional subcooling of the liquid refrigerant entering the expansion device to boost system performance. Depending on the liquid-suction heat exchanger configuration, either vapor refrigerant or liquid refrigerant is placed on a shell side of the auxiliary heat exchanger  26  into direct heat transfer interaction with the air form air moving device  32 .  
       FIG. 3  shows yet another embodiment  52  wherein a tap  55  taps a refrigerant through an economizer expansion device  58 . The tapped refrigerant from the line  55  passes through an economizer heat exchanger  54 . A main portion of the flow (or all the flow, if the tap is located downstream of the economizer heat exchanger  54 , on its main leg) also passes through the economizer heat exchanger  54  toward the main expansion device  28 . As is known, the refrigerant in the main flow is subcooled within the economizer heat exchanger  54 . A three-way valve  56  downstream of the economizer heat exchanger  54  on the tap line  55  selectively taps the refrigerant into a line  59 , if the reheat function is desired, or taps refrigerant into the by-pass line  60 , if no reheat function is desired. If the three-way valve  56  activates the reheat function, then a heat exchange portion  90  will be in the path of air having passed over the evaporator  30  as it has been driven by the air moving device  32 . While the air reheating function is provided, the temperature of the refrigerant in the heat exchange portion  90  will be reduced, as air passing over the heat exchange portion  90  is colder than refrigerant in the  90 . This is desirable, as it will improve capacity and efficiency of the refrigerant system by increasing the density and lowering the temperature of the refrigerant entering the compressor  22  through its intermediate compression port. On the other hand, if the reheat function is not desired, the three-way valve  56  is positioned to direct the refrigerant through a bypass line  60 , at which it will re-enter the line  64  at a point  62 . This refrigerant is thus returned to an intermediate compression point in the compressor  22  without having provided the reheat function. Again, an appropriate control manages the three-way valve  56  to either provide the reheat function or avoid it, depending upon sensible and latent capacity demands. It should also be noted that the valve  56  and line  60  can be removed altogether, if the reheat option to remain operational all the time. Of course, the three-way valve  56  can be substituted by a pair of the conventional valves as well as be of the metering nature by either modulation or pulsation techniques.  
       FIG. 4  shows a refrigerant system  70 , again having an economizer heat exchanger  154 . In this embodiment, the auxiliary economizer heat exchanger itself is placed in the path of the airflow from the air moving device  32 , providing the desired reheating function. At the same time, as the air temperature passing over the auxiliary economizer heat exchanger is expected to be colder than the temperature of the refrigerant passing through the heat exchanger, the temperature of the refrigerant in the heat exchanger is reduced. The refrigerant temperature reduction is beneficial to the system performance (resulting in enhanced system capacity and efficiency), as it increases refrigerant subcooling entering the main expansion device as well as reducing the temperature of the refrigerant vapor entering the compressor intermediate compression port.  
      The refrigerant flow through the economizer heat exchanger can be arranged in either counterflow or parallel flow configuration (only parallel flow arrangement is illustrated in the schematics). Also, with another variation, the tapped portion of the refrigerant can be tapped either upstream of the economizer heat exchanger (as shown in the schematics) or downstream of the economizer heat exchanger. If it is tapped upstream, then only a portion of the refrigerant exiting the condenser will pass through the main leg of the economizer heat exchanger, with the remaining portion of the refrigerant passing through the secondary leg leading to the intermediate compression port. In case of the tap located downstream of the economizer heat exchanger, the main leg of the economizer heat exchanger will carry all the flow exiting the condenser. In this case, the tapped portion of the refrigerant will enter the economizer heat exchanger on the secondary leg downstream of the economizer heat exchanger, subcooling the refrigerant in the main leg.  
      Although preferred embodiments of this invention have been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.