Abstract:
A refrigeration system includes a compressor, first and second heat exchangers, and an expansion device. A refrigerant recirculating flowpath extends sequentially downstream through the compressor, first heat exchanger, expansion device, and second heat exchanger The system includes a charge storage system. The charge storage system includes first and second refrigerant storage chambers. At least one valve is coupled to the storage chambers to permit the storage chambers to each be individually placed in alternative communication with the flowpath upstream and downstream of the expansion device.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The invention relates to refrigeration. More particularly, the invention relates to transcritical refrigeration systems used for transport or commercial refrigeration. 
         [0002]    As a natural and environmentally benign refrigerant, CO 2  (R-744) is attracting significant attention. The critical temperature for CO 2  is 87.8° F. In most air-conditioning and refrigerating operating conditions, the heat rejection occurs above this temperature so that CO 2  systems operate in transcritical mode. 
         [0003]    Different applications will require different ranges of operation (e.g., ranges of gas cooler and evaporator conditions). For example, a beverage cooler may have an essentially fixed desired interior condition (e.g., very close to 34-38° F., to avoid risk of frezing, but still provide cooling). This temperature essentially fixes the steady state compressor suction pressure. It is unlikely any operator would seek to run a beverage cooler at a different temperature. Other applications, such as transport refrigeration units (e.g., truck boxes, trailers, cargo containers, and the like), require broader capabilities. A given unit configuration may be made manufactured for multiple operators with different needs. Many operators will have the need to, at different times, use a given unit for transport of frozen goods and non-frozen perishables. An exemplary frozen goods temperature is about −10° F. or less and an exemplary non-frozen perishable temperature is 34-38° F. The operator will predetermine appropriate temperature for each of the two modes. Prior to a trip or series, the technician or driver will enter the appropriate one of the two temperatures. Other operators may have broader requirements (e.g., an exemplary overall range of −40-57° F.). 
         [0004]    Typically with variation in operating conditions, the mass flow rates and densities of the refrigerant vary greatly. For a system with fixed amount of active (circulating) charge this might cause uneven refrigerant pressure and temperature control and interfere with system performance. Additionally, the sensitivity of CO 2  to operating conditions, the relatively high pressures of operation, and the lack of two-phase state at typical charge storage points, can cause more problems. Accordingly, various charge storage systems have been proposed to permit selective withdrawal of refrigerant from circulation to allow the system to be operated more advantageously. Besides operational issues, the storage vessel, if isolated from the system, could be exposed to very high ambient temperatures. If loaded with charge, the high ambient temperatures may cause significant pressure increases. The pressure increases could cause vessel rupture. 
         [0005]    U.S. Pat. No. 7,096,679 discloses heating/cooling a reservoir to modulate the amount of refrigerant returned. Heating increases the heat load on the system, thereby making the system less efficient. The heating and cooling may increase the power consumption in the system. U.S. Pat. No. 6,385,980 discloses a flash tank economizer. If the flash tank economizer vapor line is closed for some operating conditions, then the pressure inside the flash tank may increase as described above. Other systems include an accumulator at the downstream end of the evaporator as a charge storage device. These may suffer from excessive oil build up in the bottom of the accumulator and liquid sloshing into the compressor at system startup. 
         [0006]    Thus, this present disclosure may address one to all the above problems, and provide means for regulating charge in the system over same to the entire operating envelope of typical transport and commercial applications. 
       SUMMARY OF THE INVENTION 
       [0007]    Accordingly, one aspect of the invention involves a refrigeration system including a compressor, first and second heat exchangers, and an expansion device. A refrigerant recirculating flowpath extends sequentially downstream through the compressor, first heat exchanger, expansion device, and second heat exchanger. The system includes a charge storage system. The charge storage system includes first and second refrigerant storage chambers. At least one valve is coupled to the storage chambers to permit the storage chambers to each be individually placed in alternative communication with the flowpath upstream and downstream of the expansion device. 
         [0008]    The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0009]      FIG. 1  is a partially schematic view of a first refrigeration system. 
           [0010]      FIG. 2  is a partially schematic view of a second refrigeration system. 
           [0011]      FIG. 3  is a view of a refrigerated transport unit. 
       
    
    
       [0012]    Like reference numbers and designations in the various drawings indicate like elements. 
       DETAILED DESCRIPTION 
       [0013]      FIG. 1  schematically shows a transcritical vapor compression system  20  utilizing CO 2  as working fluid (refrigerant). The system comprises a compressor  22  (e.g., a reciprocating, a scroll, or screw compressor having an electric motor), a heat rejection heat exchanger (gas cooler)  24 , an expansion device  26 , and a heat absorption heat exchanger (evaporator)  28  in sequential order along a recirculating primary flowpath. The exemplary gas cooler and evaporator may each take the form of a refrigerant-to-air heat exchanger. 
         [0014]    Airflows across one or both of these heat exchangers may be forced. For example, one or more fans  30  and  32  may drive respective airflows  34  and  36  across the two heat exchangers. The conduits along the primary refrigerant flowpath  40  include a suction line  42  extending from an outlet  44  of the evaporator  28  to an inlet  46  of the compressor  22 . A discharge line  48  extends from an outlet  50  of the compressor to an inlet  52  of the gas cooler. Additional lines  54  and  56  respectively connect the gas cooler outlet  58  to expansion device inlet  60  and expansion device outlet  62  to evaporator inlet  64 . 
         [0015]    An exemplary expansion device  26  is an electronic expansion valve (commonly identified as an EEV or EXV). An electronic expansion valve typically comprises a stepper motor attached to a needle valve to vary the effective valve opening or flow capacity. The opening of the valve may be electronically controlled by a controller  66  which may also control operation of the compressor and other system components. The controller may operate in response to input from one or more user input devices  68  (e.g., switches, electronic controls, and the like) and one or more sensors (e.g., evaporator outlet temperature and/or pressure, discharge pressure and/or temperature, ambient and controlled space temperatures). 
         [0016]    For a desired operating condition of the system, and depending on the performance of individual components of the system, there will be a particular discharge pressure at which the system operates at maximum efficiency and there will be a particular discharge pressure where the system operates at maximum capacity. While the system is going through a pulldown process, it might be advantageous that the system follow the discharge pressure which provides maximum capacity. When a steady state is reached, it might be advantageous that the system follow the discharge pressure which provides optimal efficiency (or be somewhere in between the two pressures to be optimized for a combination of efficiency and capacity). Both for operating the cycle at a given condition and for maintaining the system at the desired discharge pressure for that condition, there will be an associated optimal amount of refrigerant circulating along the flowpath  40 . Because the total system charge is fixed, a charge storage system  80  is used to store refrigerant from flowpath  40  and return refrigerant to the flowpath  40  so that the circulating charge will more closely correspond to the optimal charge as may be appropriate to maintain desired system performance. 
         [0017]    In general, as the evaporator temperature goes down, the liquid refrigerant density in the evaporator increases and greater mass of refrigerant gets stored in the evaporator. In the absence of intervention, the mass flow rate of the circulating charge decreases. At that condition it is desirable to store the least amount of refrigerant in the system  80 . Similarly, when the heat exchangers are at their highest temperatures, the evaporator will store a relatively low amount of refrigerant. To avoid overpressurizing the system  20 , it is desirable to store the most refrigerant in the storage system  80 . Thus, during system startup and pulldown it is desirable to have a maximum amount of charge in the storage system  80 . As the evaporator temperature goes down, the storage system  80  may be controlled to unload progressively more charge into the active cycle. 
         [0018]    The exemplary system includes a plurality of reservoirs  82 ,  83 , and  84  whose chambers  85 ,  86 , and  87  are fluidically coupled in parallel with each other and with the expansion device. The reservoirs may each be opened and closed to the primary flowpath  40  by valves at high and low pressure ends of the reservoirs. For purposes of illustration, each reservoir is shown having an associated first (high pressure) valve  90 ,  91 , and  92  between that reservoir&#39;s inlet  93 ,  94 , and  95  and the expansion device inlet location/condition  60 . Each reservoir further has an associated second (low pressure) valve  96 ,  97 , and  98  between a second port  99 ,  100 , and  101  of that reservoir and the expansion device outlet location/condition  62 . As is discussed further below, various of the first valves may be integrated with each other, first and second valves may be integrated with each other, or other combinations (e.g., using four-way or greater valve structures). 
         [0019]    In an exemplary method of operation, opening and closing of the first and second valves is controlled by the controller responsive to a combination of measured/sensed conditions and/or user-entered parameters (e.g., set temperatures). In the exemplary method, under normal operating conditions, each reservoir has exactly one of its two valves open while the other valve is closed. The selection of the appropriate combination of open and closed valves will determine the effective charge storage of the system  80 . 
         [0020]    For each reservoir, the amount of charge stored in the reservoir will be determined by system conditions at whichever of its first and second valves (or associated ports) is open. If the first valve is open, the reservoir will be exposed to the relatively high pressure expansion device inlet conditions. The reservoir will, therefore, hold a relatively high charge amount. If, however, the second valve is open, the reservoir will be exposed to relatively low pressure suction conditions and a relatively small amount of charge will be stored. 
         [0021]    Thus, a condition of maximum stored charge and minimum circulating charge is associated with all of the first valves being open and all of the second valves being closed. Likewise, a condition of minimum stored charge and maximum circulating charge is associated with all of the first valves being closed and all of the second valves being open. Other combinations of closed and open valves provide one or more intermediate conditions. The nature of those intermediate conditions will depend upon the relative and absolute sizes of the reservoirs. 
         [0022]    In an exemplary reservoir sizing, the relative sizes of the first and second reservoirs are selected so that the effective capacity of the second reservoir is twice that of the first reservoir (i.e., the difference in charge amount held by the second reservoir between its two conditions is twice that of the first). Similarly, the third reservoir is selected to have an effective capacity twice that of the second. The absolute sizes of the reservoirs are selected so that the combined effective capacities provide a desired overall charge storage/buffering capacity. With this exemplary combination of reservoir sizes, six evenly separated intermediate conditions may be obtained between the minimum stored charge and maximum stored charge conditions. 
         [0023]      FIG. 2  shows a more basic system with just the first and second reservoirs so that a total of four charge storage conditions can be achieved. 
         [0024]      FIG. 3  shows a refrigerated transport unit (system)  220  in the form of a refrigerated trailer. The trailer may be pulled by a tractor  222 . The exemplary trailer includes a container/box  224  defining an interior/compartment  226 . An equipment housing  228  mounted to a front of the box  224  may contain an electric generator system including an engine  230  (e.g., diesel) and an electric generator  232  mechanically coupled to the engine to be driven thereby. The refrigeration system  20  may be electrically coupled to the generator  232  to receive electrical power. The evaporator and its associated fan may be positioned in or otherwise in thermal communication with the compartment  226 . 
         [0025]    By configuring the system (either mechanically or via controller programming or hardwiring) so that one port of each reservoir is always open, the possibility of reservoir overpressure is substantially eliminated. This may allow omission of special means for preventing overpressure (e.g., separate systems for cooling the reservoirs). 
         [0026]    Although basic systems have been illustrated, more complex implementations are possible involving further features of either the reservoirs or the basic refrigeration circuit. Additional components, flowpaths, etc., may be present. 
         [0027]    One or more embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, when implemented in the retrofit/remanufacture of an existing system or a reengineering of the existing system configuration, details of the existing configuration may influence details of the particular implementation. Accordingly, other embodiments are within the scope of the following claims.