Abstract:
An expensive expansion device may be eliminated in favor of a less expensive pressure regulator in a CO 2  vapor compression system such as is used in a bottle cooler or small-capacity air conditioner, refrigerator, or other system.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    Benefit is claimed of U.S. patent application Ser. No. 60/663,960, filed Mar. 18, 2005, and entitled “High Side Pressure Regulation for Transcritical Vapor Compression System”, the disclosure of which is incorporated by reference herein as if set forth at length. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The invention relates to refrigeration. More particularly, the invention relates to beverage coolers. 
         [0003]    As a natural and environmentally benign refrigerant, CO 2  (R-744) is attracting significant attention. In most air-conditioning operating ranges, CO 2  systems operate in transcritical mode.  FIG. 1  schematically shows transcritical vapor compression system  20  utilizing CO 2  as working fluid. The system comprises a compressor  22 , a gas cooler  24 , an expansion device  26 , and an evaporator  28 . The exemplary gas cooler and evaporator may each take the form of a refrigerant-to-air heat exchanger. 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. A refrigerant flow path  40  includes a suction line extending from an outlet of the evaporator  28  to an inlet  42  of the compressor  22 . A discharge line extends from an outlet  44  of the compressor to an inlet of the gas cooler. Additional lines connect the gas cooler outlet to expansion device inlet and expansion device outlet to evaporator inlet. 
         [0004]    The major difference between transcritical and conventional operation is that heat rejection in the gas cooler is in the supercritical region because the critical temperature for CO 2  is 87.8° F. Consequently, pressure is not solely dependent on temperature and this opens additional control and optimization issues for system operation. 
         [0005]    For a fixed gas cooler discharge temperature, as the high side pressure is increased, the exit enthalpy of the refrigerant decreases, yielding a higher differential enthalpy through the gas cooler. The capacity of the gas cooler is a function of the mass flowrate of refrigerant and the enthalpy difference across the gas cooler. For a beverage cooler, the evaporator may be essentially at the cooler interior temperature. It is typically desired to maintain this temperature in a very narrow range regardless of external condition. For example, it may be desired to maintain the interior very close to 37° F. This temperature essentially fixes the steady state compressor suction pressure. 
         [0006]    For a fixed compressor suction pressure, as the high side pressure increases, the amount of energy used by the compressor increases, and the volumetric efficiency of the compressor decreases. When the volumetric efficiency of the compressor decreases, the flowrate through the system decreases. The balance of these two counteracting effects is typically an increase in gas cooler capacity as the high side pressure is increased. However, above a certain pressure the amount of capacity increase becomes very small. Because the expansion device is usually isenthalpic, the evaporator capacity will also typically increase as the high side pressure increases. 
         [0007]    The energy efficiency of a vapor compression system, the Coefficient of Performance (COP), is usually expressed as a ratio of the system capacity to the energy consumed. Because an increase in pressure typically produces both a higher capacity and a higher energy consumption, the balance between the two will dictate the overall COP. Therefore, there is typically an optimal pressure which yields the highest possible performance. 
         [0008]    An electronic expansion valve is usually used as the device  26  to control the high side pressure to optimize the COP of the CO 2  vapor compression system. An electronic expansion valve typically comprises a stepper motor attached to a needle valve to vary the effective valve opening or flow capacity to a large number of possible positions (typically over one hundred). This provides good control of the high side pressure over a large range of operating conditions. The opening of the valve is electronically controlled by a controller  50  to match the actual high side pressure to the desired set point. This pressure control strategy involves a fairly high cost valve, a sophisticated controller  50 , and a sensor  52  for measuring the high side pressure. This equipment adds a significant amount of cost to the CO 2  vapor compression system, causing the CO 2  vapor compression system to be less attractive compared to an HFC system. 
         [0009]    It is possible to use a fixed expansion device in a transcritical vapor compression system, but this approach has limitations which may cause a loss of performance or functionality. During steady state operation, a fixed expansion device (e.g., a fixed orifice or capillary tube) can work well to regulate the system high side pressure to a near optimum pressure. During pulldown, when the system is started and the evaporation temperature and pressure can be very high, the flowrate through a fixed speed and displacement compressor can become relatively high. This high flowrate can cause the high side pressure to exceed a safe limit. 
       SUMMARY OF THE INVENTION 
       [0010]    An expensive expansion device may be eliminated in favor of a less expensive pressure regulator in a CO 2  vapor compression system such as is used in a bottle cooler or small-capacity air conditioner, refrigerator, or other system. The potential for overpressurization may be reduced by using an inexpensive, multi-step fixed expansion device based on one or more solenoid valves. 
         [0011]    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  
         [0012]      FIG. 1  is a schematic of a prior art vapor compression system. 
           [0013]      FIG. 2  is a schematic of a first inventive CO 2  vapor compression system. 
           [0014]      FIG. 3  is a schematic of a second inventive CO 2  vapor compression system. 
           [0015]      FIG. 4  is a schematic of a third inventive CO 2  vapor compression system. 
           [0016]      FIG. 5  is a schematic of a fourth inventive CO 2  vapor compression system. 
           [0017]      FIG. 6  is a schematic of a fifth inventive CO 2  vapor compression system. 
           [0018]      FIG. 7  is a schematic of a sixth inventive CO 2  vapor compression system. 
           [0019]      FIG. 8  is a schematic of a seventh inventive CO 2  vapor compression system. 
           [0020]      FIG. 9  is a side schematic view of a display case including a refrigeration and air management cassette. 
           [0021]      FIG. 10  is a view of a refrigeration and air management cassette. 
       
    
    
       [0022]    Like reference numbers and designations in the various drawings indicate like elements. 
       DETAILED DESCRIPTION  
       [0023]    The current invention relates to high-side pressure optimization for a CO 2  vapor compression system. For HVAC &amp; R products which do not have broad operating envelopes, the optimal high side pressures for all operating conditions do not vary much. Therefore, a fixed expansion device (e.g., an orifice or capillary tube) can be used to regulate the high side pressure to a preset constant value for all steady state operating conditions of the CO 2  vapor compression system. The preset value should be determined such that the CO 2  vapor compression system can achieve the best overall Coefficient of Performance (COP) for the entire operating envelope. Using a fixed expansion device can significantly reduce the cost of the pressure control components in a CO 2  vapor compression system. 
         [0024]    For pulldown conditions, the compressor flowrate will be significantly higher than during steady state conditions. The high-side pressure should be optimized such that the pulldown cooling capacity of the CO 2  vapor compression system can be maximized, but the flow through the pressure regulator does not exceed the flow through the compressor (so that the system pressure becomes too great). This optimal high-side pressure for maximizing capacity is usually higher than the optimal high-side pressure for maximizing the overall COP. However, because the compressor flowrate is much higher during pulldown conditions than during steady state conditions, the expansion device may be configured to have a larger flow capacity during pulldown conditions. A simple multi-position expansion device may provide this. There are a number of ways through which this can be achieved through the use of solenoid valves to enable a two or more position pressure control system. 
         [0025]    The following examples reflect modifications of the basic system of  FIG. 1 . Accordingly, the same reference numerals are used to identify the compressor  22 , gas cooler  24 , and evaporator  28 . In any reengineering or remanufacturing situation, these components may be identical to those of the baseline system or may be further modified.  FIG. 2  shows a system  60  in which the refrigerant flow path  62  is split into two parallel branches/segments  64  and  66  between the gas cooler  24  outlet and evaporator  28  inlet. The first branch  64  has a first fixed expansion device  68 . The second branch  66  includes, in series, a solenoid valve  70  and a second fixed expansion device  72 . Although the solenoid valve  70  is shown upstream of the second fixed expansion device  72 , this order may be reversed. The exemplary solenoid valve  70  has two settings/conditions. One setting/condition is a fully closed condition in which no flow may pass along the second branch  66 . The second setting/condition is a fully open condition allowing flow to pass through the second branch  66  with a minimal pressure loss across the solenoid valve  70 . 
         [0026]    During steady state operating conditions, when the compressor flowrate is relatively low, the solenoid valve  70  is kept fully closed. During pulldown conditions, the compressor flowrate is relatively high. In order to avoid overpressurization during pulldown, the solenoid valve  70  is opened, allowing flow through the second fixed expansion device  72 . The combination of both expansion devices  68  and  72  regulates the high-side pressure to avoid overpressurization while still delivering good system performance. 
         [0027]    In operation, a pulldown condition may be detected by means of one or more temperature sensors  75  and pressure sensor  74  coupled to a controller  76  coupled to control the solenoid valve  70 . The controller  76  may also be coupled to the compressor and/or fan(s) to control their respective operation. For ease of illustration, the sensor and controller are not illustrated in the following examples although they may be present. 
         [0028]      FIG. 3  shows a system  80  wherein the refrigerant flow path  82  has two segments/branches  84  and  86  in parallel upstream of a first fixed expansion device  88 . The first branch  84  includes a solenoid valve  90 . The second branch  86  includes a second fixed expansion device  92 . During steady state operating conditions, the solenoid valve  90  is closed to prevent flow along the first branch  84 . The second branch  86  acts as a bypass with restricted flow passing through the second fixed expansion device  92  before then passing through the first fixed expansion device  88 . During pulldown conditions, the solenoid valve  90  is open, allowing an essentially unrestricted flow along the first branch  84 . A small additional flow may flow along the second branch  86 , with the combined flow then passing through the first expansion device  88 . In alternative embodiments, the first expansion device  88  may be upstream of the branching rather than downstream. Control methods and components (not shown) of this system and those discussed below may be similar to those of the system  60 . 
         [0029]      FIG. 4  shows another system  100  wherein the flow path  102  has first and second segments/branches  104  and  106  between the gas cooler and evaporator. A fixed expansion device  108  is located in the first branch  104 . A solenoid valve  110  is located in the second branch  106 . The solenoid valve  110  combines aspects of a solenoid valve and a fixed expansion device. Specifically, the open condition may still be relatively restricted compared with the open condition of the solenoid valve  90 . Therefore, the pulldown pressure drop through the solenoid valve  110  is significant and the high-side pressure of the system is controlled to the preset constant optimal value by the combination of the solenoid valve  110  and the fixed expansion device. For steady state operation, the solenoid valve  110  is fully closed and all flow passes through the expansion device  108 . 
         [0030]      FIG. 5  shows a branch-less system  120  in which, along the flow path  122 , a solenoid valve  124  and fixed expansion device  126  are located in series. The solenoid valve  124  combines aspects of the solenoid valve and a fixed expansion device differently from the valve  1   10  of  FIG. 4 . Specifically, the valve element (e.g., the solenoid plunger) of the solenoid valve  124  may have a small orifice so that its closed condition is only a partially closed condition. The open condition, however, is an essentially fully open condition with low pressure drop. Accordingly, during steady state operating conditions, the solenoid valve  124  is in its closed condition passing a relatively low flow and creating a substantial pressure drop (individually and combined with the expansion device  126 ). In the steady state condition, the solenoid valve is open, permitting the flow rate to be dictated essentially solely by the expansion device  126 . As with the other systems, the series order may be reversed. 
         [0031]      FIG. 6  shows a system  140  combining aspects of the systems  80  and  120 . Specifically, the flow path  142  has two segments/branches  144  and  146  in parallel upstream of a first fixed expansion device  148 . The first branch  144  includes a solenoid valve  150 . The second branch  146  includes a fixed expansion device  152 . The exemplary solenoid valve  150  may, similar to the solenoid valve  124 , have a closed condition that is only partially closed. During pulldown conditions, the solenoid valve  150  is open. During steady state conditions, the valve  150  is closed. In the steady state condition, there is a relatively small flow along each of the branches. During pulldown conditions, a larger flow may pass along the first branch  144 , with a residual flow along the second branch  146 . 
         [0032]      FIG. 7  shows another system  160  wherein the flow path  162  includes a solenoid valve  164  that combines solenoid valve and orifice functions. Specifically, the element of the solenoid valve  144  includes an orifice so that the closed condition is only partially closed. During steady state conditions, the valve  144  is in its closed condition with the orifice passing the relatively small flow. During pulldown conditions, the valve is open so that a larger flow is passed. 
         [0033]      FIG. 8  shows a system  180  wherein the flow path  182  includes segments/branches  184  and  186  between the gas cooler and the evaporator. A solenoid valve  188  and  190  is located in each of the branches. The elements of these solenoid valves may include orifices. Independent control over the valves may provide more than two alternative effective flow restrictions. For example, with different size orifices, the two valves provide up to four different effective restrictions. A minimal restriction may be present with both valves open. A maximal restriction may be present with both valves closed. A pair of intermediate restrictions may be achieved with one of the valves closed and the other open. To provide a more than trivial difference amongst the three least restrictive conditions, the conduit of the branches may be sized or the valve sized or additional restriction may be present so that with only one valve open there is not essentially free flow. An alternative embodiment could feature such valves in series rather than parallel. 
         [0034]    A variety of sensor and/or user inputs may be used to control the solenoid valve(s). Direct measurement of the high-side pressure may be made by the sensor  74 . When this pressure exceeds one or more associated thresholds, the controller  76  may cause the valve(s) to assume an associated relatively free-flow condition. Alternatively or in addition to high-side pressure measurement would be sensor  74 , input may be received from an air temperature sensor. The exemplary sensor  75  may be positioned to be exposed to air in or from the cooler interior (e.g., to the flow  36  upstream of the evaporator  28 ). The sensor  75  may form part of a control thermostat. Accordingly, use of such a sensor alone may permit cost savings through the elimination of the pressure sensor  52  or  74 . 
         [0035]    For fixed speed and displacement compressor, the flow through the system is a direct function of the density of the refrigerant entering the compressor and, to a lesser extent, the pressure ratio of the compressor. The inlet density is a direct function of the saturation temperature and superheat of the refrigerant. These, in turn, are direct functions of the air temperature, system size, and charge. For a simple system, these parameters may be determined in the design stage as a function of air temperature flowing through the evaporator. A correlation can be produced which matches the evaporator air temperature to the refrigerant inlet density. In operation, the solenoid valve(s) would remain in the open position until the output of the evaporator temperature sensor  75  drops below a predetermined value. When this happens, the solenoid valve or one of the solenoid valves is closed. This can be repeated for systems having multiple solenoid valves further reducing the effective expansion orifice area as the temperature drops so as to maintain a mere optimal pressure in the high pressure portion of the system. 
         [0036]    If a high-side pressure is directly measured (e.g., by the sensor  74 ) a different correlation may be used. The optimal high-side pressure may be known as a function of evaporator temperature and, optionally, the ambient temperature. The solenoid valve or valves may be actuated to maintain the pressure within certain limits. 
         [0037]      FIG. 9  shows an exemplary cooler  200  having a removable cassette  202  containing the refrigerant and air handling systems. The exemplary cassette  202  is mounted in a compartment of a base  204  of a housing. The housing has an interior volume  206  between left and right side walls, a rear wall/duct  216 , a top wall/duct  218 , a front door  220 , and the base compartment. The interior contains a vertical array of shelves  222  holding beverage containers  224 . 
         [0038]    The exemplary cassette  202  draws the air flow  34  through a front grille in the base  224  and discharges the air flow  34  from a rear of the base. The cassette may be extractable through the base front by removing or opening the grille. The exemplary cassette drives the air flow  36  on a recirculating flow path through the interior  206  via the rear duct  210  and top duct  218 . 
         [0039]      FIG. 10  shows further details of an exemplary cassette  202 . The heat exchanger  28  is positioned in a well  240  defined by an insulated wall  242 . The heat exchanger i 28  is shown positioned mostly in an upper rear quadrant of the cassette and oriented to pass the air flow  36  generally rearwardly, with an upturn after exiting the heat exchanger so as to discharge from a rear portion o the cassette upper end, a drain  250  may extend through a bottom of the wall  242  to pass water condensed from the flow  36  to a drain pan  252 . A water accumulation  254  is shown in the pan  252 . The pan  252  is along an air duct  256  passing the flow  34  downstream of the heat exchanger  24 . Exposure of the accumulation  254  to the heated air in the flow  34  may encourage evaporation. 
         [0040]    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 as a remanufacturing of an existing system or reengineering of an existing system configuration, details of the existing configuration may influence details of the implementation. Accordingly, other embodiments are within the scope of the following claims.