Flash Defrost System

Abstract

A vapour compression refrigeration system includes a compressor ( 1 ) arranged to re-circulate refrigerant through a condenser ( 2 ), an expansion device ( 4 ) and an evaporator ( 5 ). To achieve rapid thermodynamically efficient defrosting of the evaporator, hot refrigerant from the condenser is stored in a defrost receiver ( 6 ) before passing through the expansion device ( 4 ). In a defrost phase, a valve arrangement ( 7 - 10 ) forms a closed defrost circuit connecting the evaporator ( 5 ) to the defrost receiver ( 6 ) via defrost valve ( 10 ) to allow hot fluid to pass from the defrost receiver to the evaporator and liquid refrigerant in the evaporator flows to the defrost receiver ( 6 ) via drain valve ( 9 ). In a pre-defrost phase, the valve arrangement closes the fluid input to the evaporator ( 5 ) and the compressor operates to partially evacuate the evaporator before the evaporator is connected to the defrost receiver, so that flash flooding of the evaporator with hot vapour occurs. A phase change medium ( 11 ) may be included to store heat from the condenser output and return it to the evaporator during defrost. Additional heat may be supplied to the defrost liquid to further increase the defrost speed.

1 . A vapour compression refrigeration system including a compressor ( 1 ) arranged to re-circulate refrigerant through a condenser ( 2 ), an expansion device ( 4 ) and an evaporator ( 5 ), in which relatively hot refrigerant from the condenser flows through a defrost receiver ( 6 / 3 ) before passing through the expansion device, and, in a defrost phase, a valve arrangement ( 10 / 13 ,  9 ,  4 ) connects the evaporator to the defrost receiver to create a defrost circuit which allows hot fluid to pass from the defrost receiver ( 6 / 3 ) to the evaporator ( 5 ) and liquid refrigerant in the evaporator to flow to the defrost receiver, characterised in that the refrigeration system is constructed and operated such that, in a pre-defrost phase, the valve arrangement ( 9 ,  4 ) closes the fluid input to the evaporator ( 5 ) and the compressor ( 1 ) operates to partially evacuate the evaporator ( 5 ) before the evaporator is connected to the defrost receiver ( 6 / 3 ). 
     
     
         2 . A vapour compression refrigeration system according to  claim 1  in which the hot refrigerant from the condenser ( 2 ) is stored in the defrost receiver ( 6 / 3 ) before passing through the expansion device ( 4 ) and, at the commencement of the defrost phase, the stored refrigerant is admitted to the evaporator through a defrost valve ( 10 / 13 ). 
     
     
         3 . A vapour compression refrigeration system according to  claim 1  in which energy from the hot liquid refrigerant obtained from the condenser ( 2 ) is stored in a phase-change medium ( 11 / 14 ) which is in heat-exchange contact with the hot liquid, and the stored energy is released into the defrost fluid flowing through the evaporator ( 5 ) during the defrost phase. 
     
     
         4 . A vapour compression refrigeration system according to  claim 3  in which the phase-change medium ( 11 ) is contained within the defrost receiver ( 6 ). 
     
     
         5 . A vapour compression refrigeration system according to  claim 3  in which the phase-change medium ( 14 ) is included between the defrost receiver ( 3 ) and the expansion device ( 4 ). 
     
     
         6 . A vapour compression refrigeration system according to  claim 1  in which a fluid-to-fluid heat exchanger ( 15 ) is included between the defrost receiver ( 3 ) and the expansion device ( 4 ) and a heat storage fluid is circulated through the secondary of the heat exchanger to a storage reservoir ( 17 ). 
     
     
         7 . A vapour compression refrigeration system according to  claim 1  in which heating means is arranged to provide additional heat input to the hot refrigerant flowing from the condenser ( 2 ). 
     
     
         8 . A vapour compression refrigeration system according to  claim 1  which includes a plurality of evaporators ( 5 ) and in which each evaporator is associated with a respective defrost receiver ( 6 ). 
     
     
         9 . A vapour compression refrigeration system according to  claim 1  in which a pump ( 20 ) is arranged to return liquid refrigerant from the evaporator ( 5 ) to the defrost receiver ( 6 ) during the defrost phase. 
     
     
         10 . A method of defrosting a vapour compression refrigeration system including a compressor ( 1 ) arranged to re-circulate  refrigerant through a condenser ( 2 ), an expansion device ( 4 ) and an evaporator ( 5 ), in which relatively hot refrigerant from the condenser flows through a defrost receiver ( 6 / 3 ) before passing through the expansion device, and, in a defrost phase, a valve arrangement ( 10 / 13 ,  9 ,  4 ) connects the evaporator to the defrost receiver to allow hot fluid to pass from the defrost receiver ( 6 / 3 ) to the evaporator ( 5 ) and liquid refrigerant in the evaporator to flow to the defrost receiver, characterised in that the refrigeration system is constructed and operated such that, in a pre-defrost phase, the valve arrangement ( 9 ,  4 ) closes the fluid input to the evaporator ( 5 ) and the compressor ( 1 ) operates to partially evacuate the evaporator ( 5 ) before the evaporator is connected to the defrost receiver ( 6 / 3 ).