Refrigeration system

Abstract

The invention relates to a refrigeration system that includes a first and a second refrigeration circuit wherein the first refrigeration circuit uses CO 2 as the refrigerant, the second refrigeration circuit uses a second refrigerant, and the second refrigeration circuit is arranged to refrigerate the first refrigeration circuit by means of a cascade heat exchange connection. The first refrigerant is at a pressure above about 120 psig. The refrigeration system is energy efficient and is particularly useful in the food industry. The invention further relates to a method of refrigeration using the refrigerator system.

What is claimed is:  
     
         1 . A refrigeration system comprising: 
 a first closed refrigeration circuit for containing a first refrigerant comprising a high pressure side and a low pressure side, a first evaporator for evaporating a first refrigerant liquid to provide a first refrigerant vapor, a first compressor for compressing the first refrigerant vapor, a first condenser for condensing the first refrigerant vapor, and a first control valve for controlling the pressure difference between a high pressure side and a low pressure side of the first refrigeration circuit;    a second closed refrigeration circuit for containing a second refrigerant, comprising a high pressure side and a low pressure side, a second evaporator for evaporating a second refrigerant liquid to provide a second refrigerant vapor, a second compressor for compressing the second refrigerant vapor, a second condenser for condensing the second refrigerant vapor, and a second control valve for controlling the pressure difference between the high pressure side and the low pressure side of the second circuit; and    a cascade heat exchanger comprising the first condenser and the second evaporator arranged so that the first condenser is cooled by evaporation of the second refrigerant liquid in the second evaporator, wherein the first refrigerant comprises CO 2  and the first evaporator is operated at pressures above about 120 psig.    
     
     
         2 . The refrigeration system of    claim 1   , wherein the first evaporator is at a pressure of between about 120 psig and 1056 psig.  
     
     
         3 . The refrigeration system of    claim 1   , wherein the first evaporator is at a pressure of between about 120 psig and 580 psig.  
     
     
         4 . The refrigeration system of    claim 1   , wherein the first compressor compresses the first refrigerant vapor to a pressure above about 325 psig.  
     
     
         5 . The refrigeration system of    claim 4   , wherein the first compressor compresses the first refrigerant vapor to a pressure of from about 325 psig to 580 psig.  
     
     
         6 . The refrigeration system of    claim 4   , wherein the second compressor compresses the second refrigerant vapor to a pressure below about 325 psig.  
     
     
         7 . The refrigeration system of    claim 1   , further comprising an open flash economizer vessel subsequent to the first condenser to cool the first liquid refrigerant liquid to a temperature intermediate between the temperature of the first refrigerant liquid in the high pressure side and the temperature of the first refrigerant liquid in the low pressure side by evaporating part of the first liquid refrigerant to provide a first refrigerant vapor, wherein the economizer vessel is connected to an economizer port on the first compressor so that the first refrigerant vapor can be compressed by the compressor.  
     
     
         8 . The refrigeration system of    claim 1   , further comprising a heat exchanger subsequent to the first condenser to cool the first liquid refrigerant to a temperature intermediate between the temperature of the first refrigerant liquid in the high pressure side and the temperature of the first refrigerant liquid in the low pressure side by evaporating part of the first liquid refrigerant to provide a first refrigerant vapor, wherein the heat exchanger is connected to an economizer port of the first compressor so that the first refrigerant vapor can be compressed by the compressor.  
     
     
         9 . The refrigeration system of    claim 1   , wherein the second evaporator is operated at a pressure such that the temperature difference between the second evaporator and the first condenser is about 5° F.  
     
     
         10 . The refrigeration system of    claim 1   , further comprising a package refrigeration system in the first refrigeration circuit to control the pressure in the first refrigeration circuit by cooling the CO 2  when the refrigeration system is not being used.  
     
     
         11 . The refrigeration system of    claim 1   , wherein the second refrigerant is ammonia, a hydrofluorocarbon, or a hydrocarbon.  
     
     
         12 . A method of refrigerating an article comprising: 
 providing a first refrigerant of CO 2  in a first closed refrigeration circuit and second refrigerant in a second closed refrigeration circuit;    circulating the first refrigerant in the first refrigeration circuit and evaporating the first refrigerant at a pressure greater than about 120 psig to provide a first refrigerant vapor and to cool a medium, compressing the first refrigerant vapor to provide a compressed first refrigerant vapor, and condensing the compressed first refrigerant vapor to provide a first refrigerant liquid;    circulating the second refrigerant in the second refrigeration circuit and evaporating the second refrigerant to provide a second refrigerant vapor and to cool and condense the first refrigerant, compressing the second refrigerant vapor to provide a compressed second refrigerant vapor, and condensing the compressed second refrigerant vapor to provide a second refrigerant liquid; and    placing an article in the cooled medium.    
     
     
         13 . The method of    claim 12   , wherein 
 the first closed refrigeration circuit comprises a high pressure side and a low pressure side, a first evaporator for evaporating the first refrigerant liquid to provide the first refrigerant vapor, a first compressor for compressing the first refrigerant vapor, a first condenser for condensing the first refrigerant vapor, and a first control valve for controlling the pressure difference between a high pressure side and a low pressure side of the first refrigeration circuit;    the second closed refrigeration circuit for comprises a high pressure side and a low pressure side, a second evaporator for evaporating the second refrigerant liquid to provide the second refrigerant vapor, a second compressor for compressing the second refrigerant vapor, a second condenser for condensing the second refrigerant vapor, and a second control valve for controlling the pressure difference between the high pressure side and the low pressure side of the second circuit; and    a cascade heat exchanger comprising the first condenser and the second evaporator arranged so that the first condenser is cooled by evaporation of the second refrigerant liquid in the second evaporator.