Abstract:
The compressor of a transport refrigeration system is divided into two sections, with each section having a suction inlet. The refrigerant vapor from the evaporator is passed to one of the suction inlets, whereas the refrigerant vapor from a subcooler circuit is passed to the other suction inlet. A valve is operated to selectively insert or remove the subcooler as capacity requirements vary. In one embodiment, a six cylinder reciprocating compressor is used with five cylinders compressing within a first section, and a single cylinder compressing in the other section containing the subcooler circuit.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to vapor compression refrigeration systems and, more particularly, to a method and apparatus for subcooling refrigerant in a transport refrigeration system. 
     In many refrigeration systems, such as those for preserving food in supermarkets, refrigerators and the like, the load is substantially fixed and the demands of the system are substantially constant throughout the life of the system. 
     Transport refrigeration systems are different. As the types of food products that are being transported in refrigerated trucks, trailers and containers are always changing, the temperatures at which these products are desirably maintained also change. For example, one day the cargo of a truck may be bananas, with the desired temperature to be maintained at 57° degrees. On the next day, the same trailer may be hauling frozen goods, and the desired temperature to be maintained in the trailer would be 0° F. or below. They also must be able to operate in all ambient conditions as they are portable and need to be able to operate all over the world. Because of this wide range of demands, the design of a refrigeration system for a transport truck/trailer must therefore be very flexible. Thus, they must be designed to meet the maximum capacity requirements, but they are preferably designed to operate efficiently and precisely at much lower capacity requirements during most of their operating life. 
     Various marketing conditions have tended to exacerbate the problems of meeting the capacity requirements of transport refrigeration systems as discussed hereinabove. For example, because of environmental concerns, it has become necessary to abandon the use of more efficient, but environmentally undesirable, refrigerants, and to replace them with refrigerants that are less efficient. Another development that has occurred because of the need for greater cargo capacity and overall efficiencies, is a tendency to lengthen the refrigerated trailers, and also construct them with thinner side walls. 
     Current single stage compression systems have limited capacity and cannot meet the market needs as discussed hereinabove. The use of subcooling and refrigeration systems has long been used but the systems have generally been relatively complex, expensive, and difficult to maintain. Examples of such systems include those with suction liquid heat exchangers, subcoolers in condenser coils, and mechanical subcoolers using separate compressors or economizer subcoolers in multi-compressor staged systems. 
     It is therefore an object of the present invention to provide an improved transport refrigeration system. 
     Another object of the present invention is the provision in a transport refrigeration system to selectively operate at higher capacity levels in an easy to use and efficient manner. 
     Yet another object of the present invention is the provision in a transport refrigeration system for operating at a lower capacity level in a reliable and efficient manner. 
     Still another object of the present invention is the provision for transport refrigeration systems which can be readily and easily boosted in its output capacity. 
     Yet another object of the present invention is the provision for a transport refrigeration system which is economical to manufacture and effective and efficient in use. 
     These objects and other features and advantages become more readily apparent upon reference to the following description when taken in conjunction with the appended drawings. 
     SUMMARY OF THE INVENTION 
     Briefly, and in accordance with an aspect of the invention, a single compressor of a transport refrigeration system is provided with two sections, with one section being connected to the main system evaporator, and the other section being connected to a subcooling evaporator. An isolation valve and an expansion device are in the subcooler unit so as to allow for control and isolation of the subcooler when not required. 
     In accordance with another aspect of the invention, a multiple cylinder reciprocal compressor is provided with one or more cylinders being dedicated to use in the subcooler circuit, while the other cylinders are dedicated to the main evaporator circuit. 
     By yet another aspect of the invention, one or more unloading circuits are provided in the main section of the compression system such that the compressor can be unloaded during periods of low capacity demand. 
     In the drawings as hereinafter described, a preferred embodiment is depicted; however, various other modifications and alternate constructions can be made thereto without departing from the true spirit and scope of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a schematic illustration of a refrigeration system in accordance with a preferred embodiment of the invention. 
     FIG. 2 is a graphic illustration of the pH diagram of the cycle of that system. 
     FIG. 3 is a schematic illustration of an alternate embodiment of the invention. 
     FIG. 4 is a schematic illustration of yet another embodiment of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIG. 1, a vapor compression system for use in a transport refrigeration system, such as a refrigerated truck, trailer or container is shown to include a compressor  11  (shown generally in dashed lines), a condenser  12 , an expansion device  13  and an evaporator  14 , which are connected within a closed circuit to be operated in a conventional manner. 
     The compressor discharge port  16  is connected to discharge to the condenser  12  by way of the valve  17 , which can be selectively opened or closed for the purpose of isolating the compressor for service, and by the discharge check valve  15 . Downstream of the condenser  12 , a receiver  18  and an associated valve  19  may be included. 
     Expansion valve  13  is placed just upstream of the evaporator  14  and is responsive to a sensor  21  that senses the temperature of the refrigerant at the downstream end of the evaporator  14  so as to maintain a slightly superheated refrigerant condition. The superheated refrigerant then flows along the line  22  through a valve  23  to a compressor suction inlet  24 . The compressor suction inlet  24  is one of two compressor suction inlets as will be described hereinafter. 
     In order to obtain greater capacity from the compressor  11 , a subcooler  26  is provided upstream of the evaporator  14 . Upstream of the subcooler  26 , a line  27  divides into lines  28  and  29 , with line  28  passing through the subcooler  26  by way of the heat exchanger element  31  and then by way of line  32  to the expansion device  13 . A line  29  is fluidly interconnected to a valve  33 , a second expansion valve  34 , a heat exchanger element  36  and out to line  37 . A sensor  38  is interconnected to the expansion valve  34  so as to allow the expansion valve  34  to be responsive to the temperature of the refrigerant leaving the subcooler  26 . Line  37  is connected by way of valve  38  to another compressor suction inlet  39  as shown. 
     In operation, during periods in which the system demand calls for relatively low capacities, the valve  33  is in the closed position and the subcooler  26  is effectively removed from the circuit. The refrigerant flows through lines  27 ,  28 , and through the heat exchanger element  31 , to the line  32  and the expansion valve  13 . Downstream of the evaporator  14 , the refrigerant passes into the compressor suction inlet  34 , is compressed in a manner as will be described hereinafter, and is discharged at the compressor discharge port  16 . 
     During periods of operation wherein greater capacities are required, the valve  33  is opened to allow the flow of refrigerant through line  29 , the valve  33 , the expansion valve  34 , and into the heat exchanger element  36 . Because of the expansion of the refrigerant in the expansion valve  34 , the heat exchange element  34  is cooled, but with the heat exchanger element  36  being in heat exchange relationship with the heat exchanger element  31 , the transfer of heat causes a cooling of the refrigerant flowing through the heat exchanger element  31 , such that the temperature of the refrigerant entering the expansion valve  13  is subcooled. As the subcooled refrigerant passes into the evaporator, it results in a substantially greater performance of the evaporator  14 . 
     Considering in greater detail the compressor  11 , it will be seen that the compressor  11  is a multiple cylinder reciprocating compressor. Five of the six cylinders are interconnected to provide compression between the suction inlet  24  and the discharge port  16 . These are shown at  41 - 46 . Each of the cylinders has a suction valve  47 , a piston  48  and a discharge valve  49  as shown. A pair of unloaders  51  and  52  are provided to selectively connect the high pressure side back to suction as shown in order to reduce the capacity when it is not needed. Check valves  53  and  54  are also preferably provided on the high pressure side as shown. 
     Referring now to the sixth cylinder  56 , this cylinder provides compression between the compressor suction inlet  39  and the compressor discharge port  16 . It is identical to the other cylinders in that it has a suction valve  47 , a piston  48  and a discharge valve  49 , but it may well have a different displacement than the other cylinders. During periods in which the subcooler is activated within the system by the opening of the isolation valve  33 , the cylinder  56  will compress the refrigerant being discharged from the subcooler  26 , with the compressed refrigerant being mixed with that compressed by the other five cylinders of the compressor  11 . During periods in which the additional capacity is not required, the isolation valve  33  will be closed and the cylinder  56  will continue to function but will not perform any work. The isolation valve  33  could be integrated with the expansion device  34  by use of an electronic expansion valve as will be more fully discussed hereinafter. 
     When full capacity is required, all six cylinders will be compressing refrigerant and the evaporator unit will be boosted by use of the subcooled refrigerant. When fall capacity is not required, it may be reduced by turning off the subcooler or partially closing down the subcooler  26 , or by using one or both of the unloaders  51  and  52 , or a combination of these approaches. 
     Referring now to FIG. 2, the pH diagram of the system is shown when using R-404A as the refrigerant. The points  1 - 7  represent the positions on the chart which corresponds with the positions  1 - 7  within the system cycle as shown in FIG.  1 . 
     At point  1 , upstream of the expansion valve  13 , the refrigerant is at a relatively high pressure and low temperature. At point  2 , just downstream of the expansion valve  13 , the pressure is substantially reduced, and at point  3 , just upstream of the compressor suction inlet  24 , the pressure is relatively low and the temperature is substantially increased. After passing through the compressor, the temperature and pressure are increased to point  4  and after passing through the condenser at position  7 , the pressure remains almost constant but the temperature is substantially reduced. Finally, passing of the refrigerant along line  28  and through the subcooler  26  cools the refrigerant to the point  1  temperature. 
     Considering now what occurs in the other line  29  of the subcooler  26 , the passing of the refrigerant through the expansion valve  34  reduces the pressure to that at point  5 , and after passing through the subcooler  26  the temperature of that refrigerant is increased to that shown at point  6 . 
     Referring now to FIG. 3, an alternative embodiment is shown wherein the isolation valve  33  and the expansion valve  34  are replaced with an electronic expansion device  57  upstream of the subcooler  26  as shown. The electronic expansion device  57  is controlled by a controller  58  which automatically adjusts the electronic expansion device  57  toward the closed or open conditions in response to various sensed and programmed parameters. 
     On the downstream side of the subcooler  26 , the sensors  59  and  61  sense pressure and temperature, respectively, of the refrigerant in lines  37  and input those values to the controller  58 . Other inputs, such as saturation point, ambient temperature, suction pressure and discharge pressure, are input into the controller  58  by way of line  62 . 
     In response to the various input signals and the programmed software embedded therein, the controller sends signals along lines  63 ,  64  and  66  to control the electronic expansion device  57 , the unloading function, and the compressor speed, respectively, in order to optimize the system operation in a controlled and efficient manner. 
     Another embodiment of the present invention is shown in FIG. 4 wherein, a three way valve  67  is provided in line  37  and ties into line  22  by way of line  68 . The three way valve  67 , which can be controlled by solenoid  69 , would enable the six cylinder  56  to be able to use suction gas from line  37  as described hereinabove, but it also can be used to bring in suction gas from line  22 , along line  68 , to thereby permit the compressor to act as a full six cylinder machine on gas from the evaporator  14 , or as a subcooling cylinder as described hereinabove. One advantage of this arrangement is that the subcooler  26 , and all joints up to the compressor suction valve  38 , would not be under negative pressure when shut off. A possible disadvantage is the need for a three way valve, which is generally not considered to be particularly reliable. 
     While the present invention has been particularly shown and described with reference to a preferred embodiment 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 true spirit and scope of the invention as defined by the claims. For example, although the compressor has been described in terms of a six cylinder reciprocating compressor with five cylinders dedicated to one section and one cylinder to the other section, it may just as well be separated at different ratios, such as four and two, or it may have a different number of cylinders, such as one and one in a two cylinder machine, or three and one in a four cylinder machine, for example.