Abstract:
To accommodate a transcritical vapor compression system with an operating envelope which covers a large range of heat source temperatures, a high side pressure is maintained at a level determined not only by operating conditions at the condenser but also at the evaporator. A control is provided to vary the expansion device in response to various combinations of refrigerant conditions sensed at both the condenser and the evaporator in order to maintain a desired high side pressure.

Description:
TECHNICAL FIELD 
       [0001]    This invention relates generally to transport refrigeration systems and, more particularly, to a method and apparatus for optimizing the system high-side pressure in a CO 2  vapor compression system with a large range of evaporating pressures. 
       BACKGROUND OF THE INVENTION 
       [0002]    The operation of vapor compression systems with CO 2  as the refrigerant is characterized by the low critical temperature of CO 2  at approximately 31° C. At many operating conditions, the critical temperature of CO 2  is lower than the temperature of the heat sink, which results in a transcritical operation of the vapor compression system. In the transcritical operation the heat rejection occurs at a pressure above the critical pressure, and the heat absorption occurs at a pressure below the critical pressure. The most significant consequence of this operating mode is that pressure and temperature during the heat rejection process are not coupled by a phase change process. This is distinctly different from conventional vapor compression systems, where the condensing pressure is linked to the condensing temperature, which is determined by the temperature of the heat sink In transcritical vapor compression systems, the refrigerant pressure during heat rejection can be freely chosen, independent of the temperature of the heat sink However, given a set of boundary conditions (temperatures of heat sink and source, compressor performance, heat exchanger size, and line pressure drops) there is a first “optimum” heat rejection pressure, at which the energy efficiency of the system reaches its maximum value for this set of boundary conditions. There is also a second “optimum” heat rejection pressure, at which the cooling capacity of the system reaches its maximum value for this set of boundary conditions. The existence of these optimum pressures has been documented in the open literature. For example, maximum energy efficiency is attained in U.S. Pat. Nos. 6,568,199 and 7,000,413, and maximum heating capacity is attained in U.S. Pat. No. 7,051,542, all of which are assigned to the assignee of the present invention. 
         [0003]    Given a set of boundary conditions (temperature of heat source, compressor performance, heat exchanger size, and line pressure drops), the value of the optimum heat rejection pressure depends primarily on the temperature of the heat sink Conventional control schemes for CO 2  systems utilize the refrigerant temperature at the heat rejection heat exchanger outlet or the heat sink temperature or any indicator of these as the control input to control the heat rejection pressure. However, in systems designed for an operating envelope which covers a large range of heat source temperatures (e.g. −20 F to 57 F), such as transport refrigeration units, it may not be sufficient to correlate the optimum high-side pressure only to the temperature of the heat sink 
       DISCLOSURE OF THE INVENTION 
       [0004]    In accordance with one aspect of the invention, in systems having a relatively large range of heat source temperatures, the control of the system high-side pressure in a CO 2  vapor compression system is made dependent not only on the condition of refrigerant on the high pressure side (i.e. in the cooler), but also on the condition of refrigerant on the low pressure side (i.e. at the evaporator). 
         [0005]    By another aspect of the invention, in addition to temperature conditions sensed at the cooler, various sensed pressure or temperature conditions at the evaporator may be used in various combinations to determine the optimum system high-side pressure. 
         [0006]    While the present invention has been particularly shown and described with reference to the preferred mode as illustrated in the drawings, it will be understood by one skilled in the art that various changes in detail may be effected therein without departing from the spirit and scope of the invention as defined by the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a schematic illustration of one embodiment of the invention as incorporated into a transcritical refrigeration system. 
           [0008]      FIG. 2  is a schematic illustration of another embodiment thereof. 
           [0009]      FIG. 3  is a schematic illustration of yet another embodiment thereof. 
           [0010]      FIG. 4  is a block diagram illustration of the process of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0011]    Referring now to  FIGS. 1-3 , the refrigerant vapor compression system  10  will be described herein in connection with the refrigeration of a temperature controlled cargo space  11  of a refrigerated container, trailer or truck for transporting perishable items. It should be understood, however, that such a system could also be used in connection with refrigerating air for supply to a refrigerated display merchandiser or cold room associated with a supermarket, convenience store, restaurant or other commercial establishment or for conditioning air to be supplied to a climate controlled comfort zone within a residence, office building, hospital, school, restaurant or other facility. The refrigerant vapor compression system  10  includes a compression device  12 , a refrigerant heat rejection heat exchanger commonly referred to as a condenser or gas cooler  13 , an expansion device  14  and a refrigerant heat absorption heat exchanger or evaporator  16 , all connected in a closed loop, series refrigerant flow arrangement. 
         [0012]    Primarily for environmental reasons, the “natural” refrigerant, carbon dioxide is used as the refrigerant in the vapor compression system  10 . Because carbon dioxide has a low critical temperature, the vapor compression system  10  is designed for operation in the transcritical pressure regime. That is, transport refrigeration vapor compression systems having an air cooled refrigerant heat rejection heat exchanger operating in environments having ambient air temperatures in excess of the critical temperature point of carbon dioxide, 31.1° C. (88° F.), must operate at a compressor discharge pressure in excess of the critical pressure for carbon dioxide, 7.38 MPa (1070 psia) and therefore will operate in a transcritical cycle. Thus, the heat rejection heat exchanger  13  operates as a gas cooler rather than a condenser and operates at a refrigerant temperature and pressure in excess of the refrigerates critical point, while the evaporator  16  operates at a refrigerant temperature and pressure in the subcritical range. 
         [0013]    It is important to regulate the high side pressure of a transcritical vapor compression system as the high pressure has a large effect on the capacity and efficiency of the system. The present system therefore includes various sensors within the vapor compression system  10  to sense the condition of the refrigerant at various points and then control the system to obtain the desired high side pressure to obtain increased capacity and efficiency. 
         [0014]    As shown in the embodiment of  FIG. 1 , the sensors S 1 , S 2  and S 3  are provided to sense the condition of the refrigerant at various locations within the vapor compression system  10 , with the sensed values then being sent to a controller  17  for determining the ideal high side air pressure, comparing it with the actual sensed high side pressure, and taking appropriate measures to reduce or eliminate the difference therebetween. The sensor S 1  senses the outlet temperature T CO  of the condenser  13  and sends a representative signal to the controller  17 . The sensor S 2  senses the evaporator outlet pressure P EO  and sends a representative signal to the controller  17 . From those two values, the controller  17  obtains from a lookup table or from an equation/function P I =f (T S1 , P S2 ) an ideal high side pressure. In the meantime, the sensor S 3  senses the actual discharge or high side pressure P S  and sends it to the controller  17 . A controller  17  then compares the ideal pressure P I  with the sensed pressure P S  and adjusts the expansion device  14  in a manner so as to reduce the difference between those two values. Briefly, if the sensed pressure P S  is lower than the ideal pressure P I , then expansion device  14  is moved toward a closed position, and if the sensed pressure P S  is higher than the ideal pressure P I , then it is moved toward the open position. 
         [0015]    Referring now to  FIG. 2 , an alterative embodiment is shown wherein, the S 1  and S 3  values are obtained in the same manner as in the  FIG. 1  embodiment, but the S 4  sensor is placed at the inlet of the evaporator, and the values of either the evaporator inlet pressure P EI  or the evaporator inlet temperature T EI  are obtained. If the evaporator inlet pressure P IE  is sensed, then the value is sent to the controller  17  and an ideal high side pressure is obtained from a different lookup table from the  FIG. 1  embodiment. The subsequent steps are then taken in the same manner as described hereinabove with respect to the  FIG. 1  embodiment. 
         [0016]    If the sensed S 4  senses the evaporator inlet temperature T EI , then that value is sent to the controller  17  which then enters a lookup table to find the corresponding evaporator inlet pressure P EI , and the remaining steps are then taken as described hereinabove. 
         [0017]    A further embodiment is shown in  FIG. 3  wherein, rather than the condenser outlet temperature T CO , being sensed, the sensors S 5  and S 6  are provided to sense the temperature of the cooling air entering the condenser T ET  (i.e. the ambient temperature), and the temperature of the air which is leaving T LT  the condenser  13 . The controller  17  then determines the ideal high side pressure P I  on the basis of the evaporator outlet pressure P EO  and the condenser entering air temperature T ET  or on the basis of the P EO  and the condenser air leaving temperature T LT . The remaining steps are then taken in the manner described hereinabove. 
         [0018]    A functional diagram for the various sensors and the control  17  is shown in  FIG. 4 . In block  18 , the condenser outlet temperature T CO  or the condenser air entering temperature T ET , or the condenser air leaving temperature T LT  is sensed and passed to the controller  17 . In block  19 , the evaporator exit pressure P EO  or the evaporator inlet pressure P EI  or the evaporator inlet temperature T EI  is sensed and passed to the controller  17 . In block  21 , the control  17  determines the ideal high side pressure P I  by using two of the values as described above. In the meantime, a compressor discharge pressure or high side pressure P S  is sensed in block  22  and passed to the controller  17 . In block  23 , the sensed pressure P S  is compared with the ideal high side pressure P I , and the difference is passed to block  24  which responsively adjusts the expansion device  14  in the manner as described hereinabove. 
         [0019]    While the present invention has been particularly shown and described with reference to the preferred mode as illustrated in the drawings, it will be understood by one skilled in the art that various changes in detail may be effected therein without departing from the spirit and scope of the invention as defined by the claims.