Cascade refrigeration system

Abstract

Provided is a cascade refrigeration system for a transport unit. The cascade refrigeration system has a first refrigeration cycle comprising a first compressor and a first expansion valve, a second refrigeration cycle comprising a second compressor and a second expansion valve, and a cascade heat exchanger. The cascade heat exchanger comprises a condenser side fluidically coupled downstream of the first compressor and upstream of the first expansion valve, and an evaporator side fluidically coupled downstream of the second expansion valve and upstream of the second compressor. The cascade refrigeration system also comprises a pre-cooler comprising a first side and a second side. The first side is fluidically coupled downstream of the first compressor and upstream of the condenser side of the cascade heat exchanger, and the pre-cooler is configured to transfer heat between refrigerant in the first side and refrigerant in the second side.

What is claimed is: 
     
         1 . A cascade refrigeration system for a transport unit, the cascade refrigeration system comprising:
 a first refrigeration cycle comprising a first compressor and a first expansion valve;   a second refrigeration cycle comprising a second compressor and a second expansion valve;   a cascade heat exchanger comprising a condenser side fluidically coupled downstream of the first compressor and upstream of the first expansion valve, and an evaporator side fluidically coupled downstream of the second expansion valve and upstream of the second compressor; and   a pre-cooler comprising a first side and a second side, the first side being fluidically coupled downstream of the first compressor and upstream of the condenser side of the cascade heat exchanger, whereby the pre-cooler is configured to transfer heat between refrigerant in the first side and refrigerant in the second side.   
     
     
         2 . The cascade refrigeration system of  claim 1 , wherein the second side of the pre-cooler is fluidically coupled in the second refrigeration cycle, whereby the pre-cooler is configured to transfer heat between refrigerant in the first refrigeration cycle and refrigerant in the second refrigeration cycle. 
     
     
         3 . The cascade refrigeration system of  claim 1 , wherein the refrigeration system comprises two parallel paths leading from a junction, and the second side of the pre-cooler is in a different one of the two parallel paths to the evaporator side of the cascade heat exchanger. 
     
     
         4 . The cascade refrigeration system of  claim 1 , comprising a third expansion valve fluidically coupled, in a parallel fluidic connection with the second expansion valve, downstream of the second compressor and upstream of the second side of the pre-cooler. 
     
     
         5 . The cascade refrigeration system of  claim 4 , wherein the second side of the pre-cooler is fluidically coupled downstream of the third expansion valve and upstream of the second compressor. 
     
     
         6 . The cascade refrigeration system of  claim 1 , wherein the second compressor is a two-stage compressor comprising a second compressor high stage and a second compressor low stage. 
     
     
         7 . The cascade refrigeration system of  claim 6 , wherein the second compressor comprises an economiser port that opens into the second compressor at a location such that a pressure at the economiser port is between a pressure at an inlet of the second compressor low stage and an outlet of the second compressor high stage, and wherein the second side of the pre-cooler is fluidically coupled to the economiser port. 
     
     
         8 . The cascade refrigeration system of  claim 1 , wherein the first refrigeration cycle comprises a gas cooler fluidically connected downstream of the first compressor and upstream of the first side of the pre-cooler. 
     
     
         9 . The cascade refrigeration system of  claim 8 , wherein the gas cooler is configured to transfer heat between refrigerant in the first refrigeration cycle flowing through the gas cooler and an external fluid. 
     
     
         10 . The cascade refrigeration system of  claim 1 , comprising an economiser heat exchanger, the economiser heat exchanger comprising:
 a first economiser side fluidically coupled downstream of the second compressor and upstream of the second expansion valve; and   a second economiser side fluidically coupled downstream of the second compressor and upstream of the second side of the pre-cooler, whereby the economiser heat exchanger is configured to transfer heat between refrigerant in the first economiser side and refrigerant in the second economiser side.   
     
     
         11 . The cascade refrigeration system of  claim 10 , wherein the second economiser side is in a parallel fluidic connection with the second expansion valve. 
     
     
         12 . The cascade refrigeration system of  claim 1 , wherein the first refrigeration cycle comprises:
 an evaporator fluidically coupled downstream of the first expansion valve and upstream of the first compressor; and   a suction gas heat exchanger comprising a liquid line side and a suction line side, wherein the liquid line side is fluidically coupled downstream of the condenser side of the cascade heat exchanger and upstream of the first expansion valve, and the suction line side is fluidically coupled downstream of the evaporator and upstream of the first compressor, whereby the suction gas heat exchanger is configured to transfer heat between refrigerant in the liquid line side and refrigerant in the suction line side.   
     
     
         13 . The refrigeration system of  claim 12 , wherein the evaporator comprises a first fluid channel, a second fluid channel, an inlet, and a valve arrangement fluidically coupled between the inlet and the first and second fluid channels,
 wherein the first and second fluid channels are configured to pass refrigerant from the inlet through the evaporator, so that heat can be exchanged between the refrigerant in the first and second fluid channels and an external fluid that is external to the first and second fluid channels, in use, and   wherein the valve arrangement is configurable in a first configuration to fluidically couple both of the first and second fluid channels to the inlet, or in a second configuration to fluidically couple one of the first and second fluid channels to the inlet and to fluidically isolate the other of the first and second fluid channels from the inlet.   
     
     
         14 . The refrigeration system of  claim 1 , wherein the first refrigeration cycle and/or the second refrigeration cycle comprises a non-azeotropic refrigerant. 
     
     
         15 . The refrigeration system of  claim 1 , wherein the first compressor is a multi-stage compressor comprising more than one compression stage. 
     
     
         16 . The refrigeration system of  claim 15 , wherein the first compressor comprises a gas injector port that opens into the first compressor at a location such that a pressure at the gas injector port is between a pressure at an inlet of the first compressor low stage and an outlet of the first compressor high stage, and
 wherein the first refrigeration cycle comprises a gas injector valve fluidically coupled downstream of the condenser side of the cascade heat exchanger and upstream of the gas injector port.   
     
     
         17 . A transport unit comprising a cargo space for storing cargo, and the refrigeration system of  claim 1 . 
     
     
         18 . The transport unit of  claim 17 , comprising an atmosphere control system configured to control an atmosphere in the cargo space, wherein the atmosphere control system comprises the refrigeration system. 
     
     
         19 . The transport unit of  claim 17 , comprising a parts storage space for storing replacement parts for the refrigeration system, wherein the parts storage space is located outside the cargo space. 
     
     
         20 . A marine vessel comprising the refrigeration system of  claim 1 , or the transport unit of  claim 17 .