Abstract:
A refrigerant system operates in an environment defined by three distinct temperature levels, such as, for instance, the outdoor ambient temperature level, the indoor temperature level and the refrigeration temperature level. The refrigerant system is provided with an air-to-refrigerant heat exchanger located within the general indoor environment and connected to receive the flow of refrigerant from a heat rejection heat exchanger. The air-to-refrigerant heat exchanger gives off heat to the indoor air and in the process further cools the refrigerant flowing to an expansion device to thereby increase the cooling effect provided by an evaporator to the refrigeration area. Provisions are also made to partially or entirely bypass the air-to-refrigerant heat exchanger and/or the heat rejection heat exchanger, on a selective basis.

Description:
TECHNICAL FIELD 
     This invention relates generally to refrigerant systems and, more particularly, to a method and apparatus for increasing capacity of a refrigerant system by the selective use of naturally occurring temperature differences, such as between an ambient environment and a conditioned space. 
     BACKGROUND OF THE INVENTION 
     The concept of cooling a refrigerant flowing from a heat rejection heat exchanger to an expansion device in order to increase the capacity of the refrigerant system is well known. Such a refrigerant temperature reduction is most commonly accomplished in one of two ways, either by the inclusion of an economizer cycle or the use of a “liquid-suction” heat exchanger. However, each of these approaches has disadvantages. In the case of the economizer cycle, because of the need for additional components and extra complexity associated with a compressor, that has to be designed to accept vapor injection, a substantial expense is necessarily involved. 
     In the case of using a “liquid-suction” heat exchanger, the benefit is often limited, and under some circumstances, can actually reduce the cooling capacity of the refrigerant system. This occurs as the vapor entering the compressor is additionally superheated in the “liquid-suction” heat exchanger, which reduces the density of the refrigerant entering the compressor, and thus the refrigerant mass flow available for cooling. Therefore, the additional preheating of refrigerant as it enters the compressor often negates the effect of additional cooling provided by a “liquid-suction” heat exchanger. 
     There is therefore a need for increasing capacity of a refrigerant system in a simple, effective and less expensive manner. 
     DISCLOSURE OF THE INVENTION 
     In accordance with one aspect of the invention, a provision is made for including an additional air-to-refrigerant heat exchanger between an outdoor heat rejection heat exchanger and an indoor expansion device, with this heat exchanger being exposed to the indoor air temperatures to thereby further cool the refrigerant exiting the heat rejection heat exchanger, where the heat has been removed from the refrigerant by heat transfer interaction with the higher temperature ambient air, to thereby increase capacity of the refrigerant system. 
     In accordance with another aspect of the invention, a provision is made to bypass the additional air-to-refrigerant heat exchanger during periods in which the outdoor temperature is cooler than the indoor temperature. 
     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 spirit and scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of an exemplary supermarket refrigeration system with the present invention incorporated therein. 
         FIG. 2  is a graphic illustration of a P-h diagram showing the benefit of the present invention. 
         FIG. 3  is a schematic illustration of an alternative embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Shown in  FIG. 1  is an exemplary refrigeration system  11  which may be installed in a supermarket  12  with a heat rejection heat exchanger  13  being located outside to be exposed to ambient air and being fluidly connected in serial flow relationship, to an air-to-refrigerant heat exchanger  14 , an expansion device  16 , an evaporator  17 , and a compressor  18 , all of which would be typically located within the confines of the supermarket building. An air moving device such as fan  42  is associated with the heat rejection heat exchanger  13  and an air moving device such as fan  43  is associated with the evaporator  17 . In this regard, it should be understood that, generally, in larger supermarkets, the heat rejection heat exchangers associated with the large refrigeration systems are often located outdoors. The present invention is limited to such outdoor installations of the heat rejection heat exchanger  13 , and particularly to installations where the outdoor temperature is, at least at times, higher than the temperature within the general area, such as for instance customer area  19 , of the supermarket  12 . Therefore, the invention will have a greater use in warmer climates and seasons. 
     The present invention is particularly adapted to installations where, at least at times, three different, descending temperature levels are involved. These temperature levels include: the ambient temperature in which the heat rejection heat exchanger  13  resides, which may be in a temperature range of 80° F. to 120° F.; the temperature within the general indoor area  19  which would normally be in the range of 70° F. to 80° F.; and the temperature of the refrigerated zone  21  which may be in the range of 35° F. to 55° F., if non-frozen, refrigerated products are displayed therein, and in the range of −20° F. to 30° F., if frozen or chilled foods are displayed therein. The air-to-refrigerant heat exchanger  14  of the present invention therefore takes advantage of these temperature differences in order to improve performance of the refrigeration system  11 . 
     It should be noted that although the present invention references the refrigeration systems, air conditioning and heat pump systems are also within the scope and can equally benefit from the invention. As an example, if different climate-controlled zones with different temperatures levels are present within a building, a similar approach can be applied, with extra capacity obtained in the lower temperature zone due to extra cooling of the refrigerant by the air in the higher temperature zone. 
     In operation of the refrigeration system  11 , the refrigerant flows from the heat rejection heat exchanger  13  at a temperature which is typically approaching the ambient outdoor air temperature. As it enters the air-to-refrigerant heat exchanger  14 , it therefore gives off heat to the indoor environment  19  to thereby further cool the refrigerant. A fan  20  associated with the air-to-refrigerant heat exchanger  14  may be provided to enhance heat transfer interaction between the indoor air and refrigerant in the air-to refrigerant heat exchanger  14 . The temperature of the refrigerant leaving the air-to-refrigerant heat exchanger  14  would now approach the indoor air temperature, and this colder refrigerant is then passed through the expansion device  16  to the evaporator  17  for cooling the refrigerated environment  21 , such as, for example a display case or a cold room. Due to a lower temperature of the refrigerant entering the expansion device  16 , it is possible to provide a greater cooling effect in the evaporator  17  then would be possible using refrigerant with the refrigerant with temperature approximated by the ambient air temperature at the entrance to the expansion device  16 . In this way, the amount of cooling delivered to the refrigerated environment  21  by the refrigeration system  11  will be increased while the total amount of cooling delivered to the supermarket considered as a whole will remain roughly the same. That is, the amount of additional cooling provided to the refrigerated environment  21  would be approximately equal to the amount of heat dissipated into the general indoor area  19 . However, the net effect may be slightly positive, since the air-to-refrigerant heat exchanger  14  would slightly unload the outdoor heat rejection heat exchanger  13 , thus reducing power consumption for the compressor  18 . 
     Referring now to  FIG. 2 , a pressure-enthalpy (P-h) diagram is shown to illustrate the effect of the present invention. That is, after the vapor refrigerant is compressed in the vapor compression cycle from point A to point B, the heat rejection heat exchanger  13  causes the refrigerant enthalpy to be reduced from point B to point C, due to heat transfer interaction with outside ambient air. The air-to-refrigerant heat exchanger  14  then further reduces the refrigerant enthalpy from point C to point D. As the refrigerant passes through the expansion device  16  its pressure is reduced as shown by the line D-E while the refrigerant enthalpy is kept constant. Further, the refrigerant enthalpy is increased in the evaporator  17  as shown by the line E-A. It can thus be seen that the reduction in refrigerant enthalpy from point C to point D in the air-to-refrigerant heat exchanger  14  results in a greater refrigerant enthalpy change in the evaporator  17 , and thus a greater cooling potential of the refrigerant transverse the evaporator  17 , as indicated by the line E-A. 
     Recognizing that there will be periods of operation in which the outdoor temperature will be lower or substantially equal to the air temperature within the general indoor area  19 , a provision is made to selectively bypass at least a portion of refrigerant around at least portions of either the heat rejection heat exchanger  13  or the air-to-refrigerant heat exchanger  14  as shown in  FIG. 3 . 
     A bypass line  22  is provided to selectively bypass at least a portion of refrigerant around the air-to-refrigerant heat exchanger  14  to the extent permitted by operation of the refrigerant flow control devices such as valves  23  and  24  which are controlled by a control  26 . That is, if the valve  24  is closed and the valve  23  is opened, the air-to-refrigerant heat exchanger  14  will be completely bypassed by the refrigerant. Contrariwise, if the valve  23  is closed and the valve  24  is opened, then the entire flow of the refrigerant will flow through the air-to-refrigerant heat exchanger  14 . Of course, the valve  23  and  24  can be placed in intermediate positions so as to selectively determine the degree of the bypass refrigerant flow. The two valves  23  and  24  can be substituted by a single three-way valve as well. 
     Similarly, a bypass line  27 , and associated valves  28  and  29 , which are controlled by the control  26 , allow for the selective adjustment of the amount of compressed refrigerant vapor coming from the compressor  18  that bypasses the heat rejection heat exchanger  13 . For example, depending on the relative temperatures between the outdoor and indoor environments, it may be desirable for at least a portion of the refrigerant to at least partially bypass the heat rejection heat exchanger  13  and allow the air-to-refrigerant heat exchanger  14  to contribute more to the heat rejection process. This would be desirable, for instance, when heating is desired in the general indoor space  19 . 
     It has to be pointed out that the control of the heat rejection capability of either heat rejection heat exchanger  13  or air-to-refrigerant heat exchanger  14 , as well as shift of the heat flux from one heat exchanger to another, can be accomplished by the airflow control (rather than refrigerant flow control) that can be achieved, for example, by the way of a variable speed fan associated with at least one of the heat exchangers or a selective shutoff of the associated fans in the multi-fan air management system configurations. 
     Also, it has to be understood that the air-to-refrigerant heat exchanger  14  may be represented by a refrigerant line having heat transfer enhancement elements on its surface. 
     Furthermore, it has to be understood that the present invention would be particularly beneficial in the case of the CO 2  refrigerant utilized, for instance, in the supermarket refrigeration system  11 , where, in the transcritical operation, any means of capacity enhancement are highly desirable to compensate for the cycle deficiency. 
     While the present invention has been particularly shown and described with reference to a preferred and modified embodiments as illustrated in the drawings, it will be understood by one skilled in the art that various changes in detail may be made thereto without departing from the spirit and scope of the invention as defined by the claims.