Abstract:
A refrigerant system is provided with at least two stages of compression connected in series. An intercooler is positioned intermediate the two stages and is cooled by an indoor air stream. The intercooler is positioned to be in a path of air flow passing over an indoor heat exchanger, and preferably downstream of the indoor heat exchanger, in relation to this airflow. The intercooler cools the refrigerant flowing between the two compression stages as well as provides the reheat function. Benefits with regard to system performance (efficiency, capacity and reliability) are achieved with no additional circuitry or components required to provide the intercooler and reheat functions. This invention is particularly important for the CO 2  refrigerant systems operating in the transcritical cycle. Methods of control are presented for both the intercooler and reheat functions.

Description:
This application is a United States National Phase application of PCT Application No. PCT/US2006/049019 filed Dec. 21, 2006. 
     BACKGROUND OF THE INVENTION 
     This application relates to a refrigerant system, wherein the compressor is a two-stage compressor, and wherein an intercooler is provided between the two compression stages. The intercooler is placed in the air stream moving over an indoor heat exchanger, and preferably downstream of an indoor heat exchanger, in relation to the airflow, such that an intercooler heat exchanger also provides a reheat function. 
     Refrigerant systems are known and utilized to condition a secondary fluid, such as air to be delivered into a climate controlled environment. Typically, a compressor compresses a refrigerant and delivers that refrigerant to an outdoor heat exchanger, known as a condenser for subcritical applications and as a gas cooler for transcritical applications. From the outdoor heat exchanger, the refrigerant passes through an expansion device, and then to an indoor heat exchanger, known as an evaporator. 
     To obtain additional capacity, enhance system efficiency and achieve higher compression ratios, it is often the case that a two-stage compressor is provided in a refrigerant system. With a two-stage compressor, two separate compressor members or two separate compressor units are disposed in series in a refrigerant system. Specifically, for instance, in the case of a 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 stage to a suction inlet of a higher stage. If the compression ratio for the compressor system is high (which is typically the case for two-stage compression systems) and/or refrigerant suction temperature is high (which is often the case for a refrigerant system equipped with liquid-suction heat exchanger), then refrigerant discharge temperature can also become extremely high, and in many cases, may exceed the limit defined by safety or reliability considerations. Thus, it is known in the art to provide an intercooler heat exchanger (or a so-called intercooler) between the two 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. Typically, additional components and circuitry are required to provide cooling 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. This increases the cost of providing the intercooler function. 
     Another optional refrigerant system feature is a reheat circuit. In a reheat circuit, a refrigerant is passed through a heat exchanger located downstream in the path of air having passed over an evaporator. A control for the refrigerant system may then control the evaporator such that it will initially cool the air below a temperature that is desired by an occupant of the environment to be conditioned. This allows the removal of extra moisture amount from the air. The air then passes downstream over the reheat heat exchanger, and is warmed back to the desired temperature. The reheat circuit provides the ability to remove additional moisture from the air stream, when dehumidification is desired and no or little cooling is required. Typically, provision of a reheat circuit does require an additional heat exchanger, however, it does not require an additional air-moving device as it relies upon the air-moving device that is already provided to move air over the evaporator. 
     Recently, new generation refrigerants, such as natural refrigerants, are being utilized in refrigerant systems. One very promising refrigerant is carbon dioxide (also known as CO 2  or R744). Particularly with CO 2  refrigerant systems, the intercooler becomes even more important as these systems tend to operate at high discharge temperatures due to high operating pressures, frequent use of liquid-suction heat exchanger, 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 of the circuitry and components associated with the intercooler makes the provision of an intercooler less desirable. 
     Thus, it is desirable to provide an intercooler for a multi-stage compressor refrigerant system, and particularly for a CO 2  refrigerant system, as well as a reheat function, that essentially does not require any additional circuitry or components beyond the intercooler itself. 
     SUMMARY OF THE INVENTION 
     In a disclosed embodiment of this invention, a refrigerant system incorporates a multi-stage compressor. An intercooler is provided between at least two of the compression stages connected in series. The intercooler is positioned to be subjected to the airflow passing over an indoor heat exchanger. Preferably, the intercooler is positioned downstream of the indoor heat exchanger, with respect to the airflow delivered to a conditioned space. Thus, the intercooler heat exchanger may also selectively provide the reheat function, preferably at operating conditions when dehumidification with little or no cooling is desired. The reheat function and the intercooler function may be activated on demand. For instance, a refrigerant bypass around the intercooler may be provided when the intercooler function is not required and/or an air damper may be installed to bypass airflow around the intercooler in cases when the reheat function is not needed. 
     Positioning the intercooler in the indoor air stream allows for a single heat exchanger to provide both the intercooler and reheat functions. Moreover, by positioning the intercooler downstream of the indoor heat exchanger, an additional air-moving device associated with the intercooler is not required. Instead, the air-moving device that is already associated with the evaporator also moves air across the intercooler heat exchanger. In this way, both a reheat function and an intercooler function are provided with only the provision of a single heat exchanger. 
     In this invention, when the refrigerant system is operating in a dehumidification mode, an indoor air-moving device that passes air over the indoor heat exchanger also cools the refrigerant flowing in the intercooler between the lower and higher compression stages. The intercooler increases refrigerant system capacity and improves efficiency, since the compressor discharge temperature is reduced, and the outdoor heat exchanger (a condenser or a gas cooler) is capable to cool refrigerant to a lower temperature, providing a higher cooling potential in the evaporator. 
     Additionally, if the system operates in a transcritical cycle, such as a CO 2  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 the value providing an optimum performance level. Thus, efficiency and capacity of the refrigerant system will be enhanced even further. 
     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. 1A  shows a schematic of an inventive refrigerant system. 
         FIG. 1B  shows an alternative arrangement. 
         FIG. 2  shows an intercooler refrigerant bypass arrangement. 
         FIG. 3  shows an intercooler air bypass arrangement. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A refrigerant system  20  is illustrated in  FIG. 1A  having a lower stage compressor  22  and a higher stage compressor  24 . While only two stages are shown, additional stages may also be incorporated in series in this invention. Also, instead of separate compressors connected in sequence, a multi-stage compressor arrangement can be employed and equally benefit from the present invention. For instance, the two separate compression members ( 22  and  24 ) may represent different banks of cylinders connected in series for a reciprocating compressor. As known, refrigerant compressed by a lower stage to an intermediate pressure is delivered from a discharge outlet of this lower stage to a suction inlet of the higher stage. An intercooler  26  is positioned between the two stages to accept refrigerant from a discharge outlet of the lower stage  22 , cool it by a secondary media (fluid), such as air to be delivered to a conditioned space blowing over external heat transfer surfaces of the intercooler  26  during heat transfer interaction with the refrigerant, and deliver it downstream to a suction inlet of the higher stage  24 . Again, if additional stages of compression are provided, additional intercoolers may also be positioned between those stages. 
     Refrigerant is compressed at the low stage compressor  22  from a suction pressure to an intermediate pressure, flows through the intercooler  26 , where it is cooled by a secondary media such as indoor air, compressed from an intermediate pressure to a discharge pressure at the higher stage compressor  24 , and then delivered to an outdoor heat exchanger (a condenser for subcritical applications or a gas cooler for transcritical applications)  30 . From the outdoor heat exchanger  30 , the refrigerant passes through an expansion device  32 , where it is expanded from a pressure typically approximating the discharge pressure to a pressure approximating the suction pressure, while its temperature is reduced, and then flows to an evaporator  34 . From the evaporator, refrigerant returns to the lower stage compressor  22 . 
     An air-moving device  36  blows air over external surfaces of the evaporator  34 . That air is delivered into a climate controlled environment  40 . As can be appreciated in  FIG. 1A , the intercooler  26  is positioned to be in the path of air having flowed over the evaporator  34 , and driven by the air-moving device  36 . 
     As is known, a control for the refrigerant system  20  may control the condition of the refrigerant in the evaporator  34  such that it cools this air to a temperature below that desired by an occupant of the climate controlled environment  40 . In this manner, an additional amount of moisture may be removed from the air, as desired. The air then serially passes over the intercooler  26 , and can be heated back to the temperature that is desired in the conditioned environment  40 . As the refrigerant in the intercooler heats the air delivered to the conditioned environment  40 , the refrigerant itself is cooled, enhancing performance (capacity, efficiency and reliability) of the refrigerant system  20 . Thus, both the reheat function and the intercooler function are provided with only the requirement of the single additional heat exchanger  26 . 
     When the refrigerant system  20  is operating in the cooling mode, the intercooler  26  increases system capacity and efficiency, since the compressor discharge temperature is reduced and the outdoor heat exchanger  30  (once again, a condenser or a gas cooler) is capable to cool refrigerant to a lower temperature, providing a higher cooling potential for the refrigerant entering the evaporator  34 . Required compressor power is also reduced as heat is removed from the compression process, and the outdoor heat exchanger  30  operating pressure is reduced as well. Additionally, if the refrigerant system  20  operates in a transcritical cycle, such as a CO 2  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. Additionally, in both subcritical and transcritical cycles, the temperature of the refrigerant discharged from the higher compression stage  24  is reduced, improving reliability of the compressor. Thus, performance (efficiency and capacity) of the refrigerant system  20  is increased and compressor reliability is improved. 
     The present invention is particularly useful in heat pumps that utilize CO 2  as a refrigerant, since the CO 2  refrigerant has a high value of a polytropic compression exponent, and 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. 
     It should be noted that this invention is not limited to the system shown in the  FIG. 1A , as the actual refrigerant system may include additional components, such as, for example, a liquid-suction heat exchanger, a reheat coil, an additional intercooler, an economizer heat exchanger or a flash tank. Also, the individual compression stages may include several compressors arranged in tandem. The compressors can be of variable capacity type, including variable speed and multi-speed configurations. Further, the compressors may have various unloading options, including intermediate pressure to suction pressure bypass arrangements. On the other hand, the compressors may be unloaded internally, as for example, by separating fixed and orbiting scrolls from each other on an intermittent basis. These system configurations are also not limited to a particular compressor type and may include scroll compressors, screw compressors (single or multi-rotor configurations), reciprocating compressors (where, for example, some of the cylinders are used as a lower compression stage and the other cylinders are used as a higher compression stage) and rotary compressors. The refrigerant systems may also consist of multiple separate circuits. The present invention would also apply to a broad range of systems, for example, including mobile container units, truck-trailer and automotive systems, packaged commercial rooftop units, supermarket installations, residential units, environmental control units, etc. 
     Also, it should be understood that, in some cases, it would be beneficial to position the intercooler  26  upstream of the evaporator  34 , with respect to the indoor airflow. For instance, if the evaporator  34  has an undesirably low sensible heat ratio (the ratio of sensible and latent capacities) or if the capacity of the evaporator  34  needs to be increased, the intercooler  26  may be positioned upstream of the evaporator  34 , in these applications, as shown in  FIG. 1B . 
       FIG. 2  exhibits another embodiment of the present invention, where a three-way valve  48  is positioned between the lower compression stage  22  and the higher compression stage  24  and allows for a selective refrigerant bypass of the intercooler  26  when the intercooler or/and reheat functions are not required. In such cases, the control (not shown) for the refrigerant system  20  moves the three-way valve  48  to a bypass position, so that the refrigerant flows directly from the lower compression stage  22  to a bypass line  52 , through the three-way valve  48 , to a bypass line  54  and then to the higher compression stage  24 . Therefore, in this mode of operation, the intercooler  26  is eliminated from an active refrigerant circuit. On the other hand, when the intercooler function or/and the reheat function is required, the three-way valve  48  is moved to a conventional position, so that the refrigerant flow through intercooler  26  (as well as interconnecting lines  46  and  50 ) is allowed, and the refrigerant system  20  resumes its normal operation as described above. Further, a check valve  44  may be placed on the interconnecting line  50 , to prevent refrigerant migration when the intercooler  26  is eliminated from an active refrigerant circuit. 
     The three-way valve  48  can be replaced by a pair of conventional valves, as known in the art. Further, if a more flexible control is required for the reheat or/and intercooler functions, the three-way valve  48  (or a substituting pair of conventional valves) may be operated in pulsation or modulation mode by a control for the refrigerant system  20 . 
       FIG. 3  shows yet another embodiment of the present invention. In this design, an indoor air baffle (or damper) is positioned between the evaporator  34  and intercooler  26 , with respect to indoor airflow. If the damper  62  is inactive (position  100 ), both reheat and intercooler functions are engaged, since indoor air stream flows over the external surfaces of the intercooler  26 . In cases when the reheat function is not required, the indoor air baffle  62  may be actuated by the control (not shown) for the refrigerant system  20 . When the indoor air baffle  62  is raised, it prevents the indoor air from flowing over the external surfaces of the intercooler  26 , thus depressing the reheat function. Even though no active convection heat transfer is taking place in the intercooler  26  with the indoor air baffle  62  actuated, some limited intercooler function will be still provided, since the intercooler  26  is positioned within the cold section of the refrigerant system  20 . 
     Moreover, if a more flexible control is required for the reheat or intercooler functions, the indoor air baffle  62  may be controlled continuously or discretely to a number of intermediate positions between fully actuated and non-actuated positions. 
     Also, it has to be understood that the indoor air baffle  62  can be replaced by other means of the indoor airflow control, such as, for instance, a stack of louvers or any other technique known in the art. 
     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.