system 300 has a line 308 that extends from the reservoir 16 the sub cooling refrigerant directed in the line 308 is expanded by expansion stage 304 such that its pressure is reduced the sub cooling refrigerant is then put in heat exchange with the cooling refrigerant in heat exchange stage 306 so as to absorb heat from the cooling refrigerant and thus liquefy the cooling refrigerant for its subsequent mixture with the defrost refrigerant the sub cooling refrigerant is then fed to the compression stage 12 also in fig4 a valve 400 is shown at the outlet of the dedicated compressor 12 a the valve 400 is provided so as to ensure that the line 106 at the outlet of the compressor 12 a maintains sufficient refrigerant pressure in fig5 a a sub cooling system 300 u2032 is similar to the sub cooling system 300 of fig5 a but with the valve 202 positioned upstream of the heat exchanger 306 in fig5 b a sub cooling system 300 u2033 has the line 112 u2032 mixing the defrost refrigerant to the cooling refrigerant upstream of the heat exchanger 306 in fig5 c a sub cooling system 300 u2032 u2033 collects sub cooling refrigerant downstream of the heat exchanger 306 it is pointed out that line 112 u2032 can mix defrost refrigerant to the cooling refrigerant downstream or upstream of the heat exchanger 306 as is illustrated other sub cooling configurations are also possible it is obvious that the control of valve operation is preferably fully automated the valve operation for controlling the defrost of evaporators 20 namely the control of valves 114 116 118 and 120 is fully automated the defrosting of one of the evaporators 20 can be stopped according to.