Abstract:
A refrigerant system is provided with a multi-stage compression system. An intercooler is positioned between at least two compression stages to cool a refrigerant, by heat transfer interaction with a secondary fluid, after it has been compressed in the lower compression stages to some intermediate pressure. The intercooler enhances refrigerant system performance, improves compressor reliability, and extends operational envelope. Further, at least one economizer circuit is incorporated into the refrigerant system that returns the economized refrigerant flow at the location between at least two compression stages.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This application relates to a refrigerant system with a multi-stage compressor that combines the benefits of an intercooler heat exchanger and an economizer cycle. In particular, this application relates to a refrigerant system operating, at least for a portion of the time, in a transcritical cycle. 
         [0002]    Refrigerant systems are known, and are utilized to condition a secondary fluid. As an example, an air conditioning system cools and dehumidifies air being delivered into a climate controlled environment. 
         [0003]    A basic refrigerant system includes a compressor compressing refrigerant and delivering that refrigerant through a discharge line downstream to a first heat exchanger, a so-called condenser for subcritical applications or a gas cooler for transcritical applications. In the first heat exchanger, the heat is removed from the refrigerant by a secondary media, such as ambient air. From the first heat exchanger, refrigerant passes through an expansion device, where it is expanded to a lower pressure and temperature, and then through a second heat exchanger or so-called evaporator, where the heat is transferred to the refrigerant from other secondary fluid, such as indoor air, to be conditioned and delivered to a climate controlled environment. The refrigerant is then returned to the compressor to repeat the cycle. 
         [0004]    To obtain additional capacity, enhance system efficiency and achieve higher compression ratios, it is often the case that a multi-stage compressor is provided in a refrigerant system. With a multi-stage compressor, several separate compression members or several separate compressor units are disposed in series in a refrigerant system. Specifically, for instance, in the case of a two-stage reciprocating compressor, two separate compression members may be represented by different banks of cylinders connected in series. Refrigerant compressed by a lower stage to an intermediate pressure is delivered from a discharge outlet of this lower compression stage to the suction inlet of the higher compression stage. For a multi-stage compression system, this process is repeated. If the compression ratio for the compressor system is high (which is typically the case for multi-stage compression systems) and/or refrigerant suction temperature is high (which is often the case for a refrigerant system equipped with a liquid-suction heat exchanger), then refrigerant discharge temperature can also become extremely high, and may exceed the limit defined by safety or reliability considerations. 
         [0005]    Thus, it is known in the art to provide an intercooler heat exchanger (or a so-called intercooler) between the high and low compression stages to extend the operational envelope and/or improve system reliability. In the intercooler, refrigerant flowing between the two compression stages is typically cooled by a secondary fluid. Quite often, additional components and circuitry are required to provide cooling of the refrigerant in the intercooler. As an example, a fan or pump is supplied to move a secondary cooling fluid from a cold temperature source to cool the refrigerant in the intercooler. 
         [0006]    Another option that is known in the refrigerant art is the use of an economizer cycle. An economizer cycle taps a portion of refrigerant from a liquid refrigerant line and expands the tapped refrigerant to some intermediate (between suction and discharge) pressure. The partially expanded tapped refrigerant is then passed through a heat exchanger in heat exchange relationship with the liquid refrigerant flow circulating through the main refrigerant circuit and prior to entering main expansion device. In this manner, the main refrigerant flow in the liquid line is cooled, while the tapped portion of refrigerant flow is evaporated and typically superheated. The tapped refrigerant is then returned to an intermediate pressure point in a compression system. As also known, a flash tank separating vapor and liquid phases of refrigerant may be used as the economizer heat exchanger and essentially provide similar benefits to the refrigerant system performance and operation. 
         [0007]    The combination of the intercooler heat exchanger and the economizer cycle has not been fully realized for multi-stage compression systems and would be especially beneficial in modern refrigerant systems that are operating, at least for portion of the time, in transcritical cycle and utilizing natural refrigerants such as carbon dioxide (also known as CO 2  or R744). 
         [0008]    In particular, with the CO 2  refrigerant systems, the intercooler heat exchanger and the economizer cycle become even more important, as these systems tend to operate at high discharge temperatures due to high operating pressure ratios, and, in general, by the transcritical nature of the CO 2  cycle, as well as a high value of the polytropic compression exponent for the CO 2  refrigerant. However, the additional cost and complexity of the circuitry and components associated with the intercooler and economizer, makes the provision of the intercooler less feasible. However, it become desirable to provide proper intercooler and economizer configurations for multi-stage compressor refrigerant systems, and particularly for CO 2  refrigerant systems, for the reasons described above. 
       SUMMARY OF THE INVENTION 
       [0009]    In a disclosed embodiment of this invention, a refrigerant system is provided with at least two compression stages connected in series. The refrigerant is progressively compressed to a higher pressure while flowing from a lower compression stage to a higher compression stage. At least one intercooler is placed between at least the two compression stages to cool the refrigerant after it exits the lower stage and before it enters the higher stage. In addition, the refrigerant system incorporates at least one economizer cycle, with the tapped portion of refrigerant returned from the economizer branch to an intermediate compression point between the higher and lower compression stages. In one embodiment, there are at least three compression stages connected in series, and the tapped portion of refrigerant is returned to a point in the compression cycle between the two compression stages that are different from the compression stages between which the intercooler is located. In this embodiment, the intercooler is preferably positioned downstream of the return point of the tapped economized refrigerant. In another embodiment, the intercooler and the economizer branch tapped refrigerant return point are positioned between the same two compression stages, with the economized refrigerant return point being preferably located downstream of the intercooler heat exchanger. 
         [0010]    Preferably, the intercooler heat exchanger is positioned between the higher compression stages of the multi-stage compression system (with more than two compression stages), where refrigerant temperatures have reached higher values, allowing for larger temperature differentials between the refrigerant and a secondary fluid, enhancing heat rejection capability, and improving performance of the refrigerant system. This is especially beneficial when ambient air is utilized directly or indirectly (e.g., through auxiliary loops with an intermediate fluid, such as city water) as a secondary fluid, in particular, at high ambient temperatures. The locations for the intercooler and the economizer circuit can be interchanged, with the intercooler positioned between the lower compression stages and the economizer circuit positioned between the higher compression stages, depending on the temperature of the secondary fluid utilized in the intercooler and capacity vs. efficiency tradeoff for the economizer circuit. 
         [0011]    On the other hand, strategically positioning the economizer circuit between the lower compression stages allows for larger temperature differentials in the economizer heat exchanger and thus for higher refrigerant cooling potential in the evaporator and the refrigerant system performance (capacity and/or efficiency) enhancement. Also, the colder refrigerant injected between the compression stages further reduces discharge temperature, improves reliability of the entire compression system, extends an operational envelope for the refrigerant system and enhances evaporator dehumidification capability. Lastly, positioning the economizer circuit between lower compression stages allows for a larger step in refrigerant system unloading strategy, which is desired in most of the applications. 
         [0012]    This is especially important in case of transcritical operation, where the high side temperature and pressure are independent from each other. In the transcritical operation, the discharge pressure is not limited by the discharge temperature anymore and can be adjusted to the value providing an optimum performance level. Thus, in such circumstances, the refrigerant system efficiency and capacity can be enhanced even further by optimizing the discharge pressure. 
         [0013]    In another embodiment, both intercooler and economizer circuit are positioned between the same compression stages. Once again, the relative position of the intercooler and the economizer circuit, with respect to refrigerant flow primarily depends on the temperature of the secondary fluid utilized in the intercooler and capacity-efficiency tradeoff for the economizer circuit. 
         [0014]    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 
         [0015]      FIG. 1  shows a first schematic view of a refrigerant system incorporating the present invention. 
           [0016]      FIG. 2  shows a second schematic view of a refrigerant system incorporating the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0017]    A refrigerant system  20  is illustrated in  FIG. 1 . Three stages of compression  24 ,  26 , and  28  are positioned in series within the refrigerant system  20  to progressively compress refrigerant from suction to discharge pressure. Although a multi-stage compressor system is represented by separate compressor units that are disposed in series, as shown in  FIG. 1 , separate compression members can be utilized instead of some or all of the compressor units. Specifically, for instance, in the case of a three-stage reciprocating compressor, the three separate compression members may represent different banks of cylinders connected in series. Refrigerant, compressed by the first stage from a suction pressure to a first intermediate pressure, is delivered from a discharge outlet of this first stage to the suction inlet of the second stage. Refrigerant vapor is compressed by the second stage to a second intermediate pressure and delivered from a discharge outlet of this second stage to the suction inlet of the third stage. Lastly, refrigerant, compressed by the third stage to a discharge pressure, is delivered from a discharge outlet of this third stage to a discharge line of a refrigerant system. An intercooler heat exchanger  30  is positioned between the second and third compression stages  26  and  28 . Secondary fluid, such as air blown by a fan  32 , passes over the intercooler  30  to cool the refrigerant. 
         [0018]    Cooling refrigerant in the intercooler  30  increases system capacity and efficiency, since the compressor discharge temperature is reduced and the first or outdoor heat exchanger  34  (a condenser in the subcritical cycle and a gas cooler in the transcritical cycle) will be capable of cooling refrigerant to a lower temperature, eventually providing a higher cooling potential for the refrigerant entering the evaporator  50 . Compressor power is also reduced, as heat removed from the compression process decreases the operating pressure of the outdoor heat exchanger  34 . Additionally, if the refrigerant system  20  operates in a transcritical cycle, where the high side temperature and pressure are independent from each other, the discharge pressure is not limited by a discharge temperature anymore and can be adjusted to a value corresponding to an optimum performance level. Moreover, in both subcritical and transcritical cycles, the temperature of the refrigerant discharged from the highest, third compression stage  28  is reduced, improving overall reliability of the compression system. Thus, performance (efficiency and capacity) of the refrigerant system  20  is increased and compressor reliability is improved. 
         [0019]    The present invention is particularly useful in refrigerant systems that utilize CO 2  as a refrigerant, since CO 2  refrigerant has a high value of a polytropic compression exponent, and the discharge operating pressures and pressure ratios of such systems can be very high, promoting higher than normal discharge temperatures. Still, the invention would extend to refrigerant systems utilizing other refrigerants. 
         [0020]    Preferably, the intercooler heat exchanger  30  is positioned between the higher compression stages, such as the compression stages  26  and  28  in  FIG. 1 , where refrigerant temperature have reached the higher values, allowing for the larger temperature differentials between the refrigerant and secondary fluid, enhanced heat rejection capability, and superior performance of the refrigerant system  20 . This is especially beneficial when ambient air is utilized directly or indirectly (e.g., through auxiliary loops with an intermediate fluid, such as city water) as a secondary fluid, in particular, at high ambient temperatures. 
         [0021]    From the third compression stage  28 , the refrigerant passes through the outdoor heat exchanger  34 , and then to an economizer heat exchanger  36 . As known, a tapped portion of refrigerant in a tap line  38  is tapped from a liquid line  40 . The tapped refrigerant in the tap line  38  passes through an economizer expansion device  42 , where it is expanded to some intermediate (between suction and discharge) pressure. During the expansion process in the economizer expansion device  42 , the temperature of the tapped portion of refrigerant is reduced as well. Therefore, the tapped expanded refrigerant flowing through the economizer heat exchanger  36  is able to cool refrigerant in the liquid line  40 . Although, for illustration simplicity, the two refrigerant streams are shown flowing in the same direction, in this embodiment, in practice, it is desirable to arrange the two flows in the counterflow configuration. The tapped portion of refrigerant is evaporated and typically superheated, during heat transfer interaction with the liquid refrigerant in the liquid line  40  in the economizer heat exchanger  36 , and is returned through a vapor injection refrigerant line  44  to an intermediate point  46  between the first and second compression stages  24  and  26 . 
         [0022]    Downstream of the economizer heat exchanger  36 , refrigerant in the liquid line  40 , having been cooled to a lower temperature in the economizer heat exchanger  36  and therefore having higher cooling potential, passes through a main expansion device  48 , where it is expanded to a pressure approximated the suction pressure, and then through an evaporator  50 , where it conditions a secondary fluid supplied to a climate controlled environment, while the refrigerant is evaporated and typically superheated prior to entering the compression system. From the evaporator  50 , the refrigerant is returned to the first compressor stage  24  to repeat the cycle. 
         [0023]    As known, in a majority of the cases, the economizer cycle allows for enhanced performance (capacity and/or efficiency), reduced discharge temperature, improved reliability, more flexible unloading strategy and better dehumidification capability. Strategically positioning the economizer circuit return line  44  between the lower compression stages, such as the compressor stages  24  and  26  in  FIG. 1 , allows expansion of the tapped portion of refrigerant in the economizer expansion device  42  to a lower intermediate pressure, and thus obtaining larger temperature differentials in the economizer heat exchanger  36  between the refrigerant in the liquid line  40  to be cooled and the tapped portion of refrigerant. These higher temperature differentials in turn allow for lower temperatures of the refrigerant in the liquid line  40  and higher cooling potential in the evaporator  50 . Therefore, the system performance (capacity and/or efficiency), as well as its dehumidification capability, can be increased significantly, by locating the vapor injection line  44  of the economizer cycle between lower compression stages. Also, the colder refrigerant injected between the compression stages  24  and  26  further reduces discharge temperature, improves reliability of the entire compression system and extends the operational envelope for the refrigerant system  20 . Once again, this is especially important in case of transcritical operation, where the high side temperature and pressure are not directly related to each other, i.e. the discharge pressure is not limited by the discharge temperature anymore and can be adjusted to the value providing an optimum performance level. Thus, in such circumstances, refrigerant system efficiency and capacity can be enhanced even further by optimizing the discharge pressure. Additionally, it is beneficial in situations where the intercooler heat exchanger  30  alone is not capable of performing the desired function and assuring efficient and reliable operation of the refrigerant system  20 . Lastly, positioning the vapor injection line  44  between the lower compression stages  24  and  26  allows for a large step in refrigerant system unloading, which is desired in most of the applications. 
         [0024]    By incorporating the intercooler heat exchanger  30  and the economizer cycle, and utilizing strategic locations for both enhancement option, the present invention provides maximum benefits in performance (capacity and/or efficiency), reliability, operational envelope extension, unloading capability, dehumidification flexibility and ability to achieve precise control over the temperature and humidity in the conditioned environment 
         [0025]    Although only three compression stages are shown in  FIG. 1 , refrigerant systems having more than three compression stages, with the economizer circuit preferably positioned between the lower compression stages and the intercooler heat exchanger positioned between the higher compression stages, can equally benefit and are within the scope of the present invention. Further, depending on the temperature of the fluid utilized to cool refrigerant in the intercooler  30  (to obtain an overall counterflow configuration) and a tradeoff between refrigerant system capacity and efficiency related to the economizer circuit, the locations of the intercooler and the return point of the vapor injection line  44  can be interchanged, with the intercooler  30  being positioned between the lower compression stages and the economizer circuit positioned between higher compression stages. 
         [0026]      FIG. 2  shows another embodiment  60 , wherein the refrigerant system incorporates a higher stage and a lower stage of compression  62  and  64  respectively, with the intercooler heat exchanger  66  and the return point  68  for the vapor injection line  44  of the economizer branch both being positioned intermediate the two compression stages. As shown in this embodiment, the return point  68  of the vapor injection line  44  is located downstream of the intercooler  66 , with respect to refrigerant flow. Further, in this embodiment, the tap line  70  for tapping the portion of refrigerant to pass through the economizer heat exchanger  36  is positioned downstream of the economizer heat exchanger  36 . The economizer circuit and economizer expansion device  72  operate as in the  FIG. 1  embodiment. Also, rather than utilizing the fan  32  of  FIG. 1 , a fluid conduit  80  is used to cool the refrigerant in the intercooler heat exchanger  66 . The fluid in the conduit  80  can be supplied, for instance, by a pump (not shown). Although the refrigerant system  60  shown in  FIG. 2  has less flexibility and potential for operation enhancement, in comparison to the  FIG. 1  embodiment, the benefits obtained from the combination of the intercooler  66  and economizer circuit are still significant. Obviously, the location of the return point  68  of the vapor injection line  44  can also be upstream of the intercooler heat exchanger  66 , with respect to refrigerant flow, and depends on the temperature of cooling fluid in the conduit  80 , in order to provide most efficient overall conterflow configuration. Refrigerant systems with more than two compression stages can equally benefit from this embodiment, where the intercooler heat exchanger  66  and the economizer circuit positioned between the same compression stages. 
         [0027]    It should be pointed out that many different compressor types could be used in this invention. For example, scroll, screw, rotary, or reciprocating compressors can be employed. The use of a lower and upper compression stage can be combined within a single compressor, where the vapor injection would take place at the intermediate location in the compression cycle for this compressor. Alternatively, the upper and lower compression stages can be represented by a separate compression elements, with the vapor injection or intercooling taking place between the stages. The compression elements can be separate compressor units or the compression elements can be a part of a single compressor, as it is the case for a reciprocating compressor where each compression element can be represented by a single bank of cylinders for this reciprocating compressor. The refrigerant systems that utilize this invention can be applied in many different applications, including, but not limited to, air conditioning systems, heat pump systems, marine container units, refrigerated truck-trailer systems, and supermarket refrigeration applications. 
         [0028]    Although a preferred embodiment of this invention has 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.