a freon sub cooler 126 utilizing liquid carbon dioxide for sub cooling freon expansion valve 128 and freon evaporator chiller 130 also deep cooling liquid carbon dioxide as compressor 122 must handle intake pressures of the refrigerant at the equilibrium pressure in the range of about minus 60 u00b0 f and discharge pressures at the equilibrium pressure in the range of about 100 u00b0 f compressor 122 must be capable of at least about 10 compression ratios and capable of operating at vacuum or near vacuum intake conditions very low temperatures are possible in the evaporator as the sub cooler 126 cools the freon or other refrigerant to about 0 u00b0 f the nominal temperature of liquid carbon dioxide 12 a in vessel 11 before freon expansion in valve 128 without this sub cooling the deep cooling capacity of system 121 would be small line 19 and line 131 supply the liquid carbon dioxide 12 a from vessel 11 with the flow controlled by valve 132 carbon dioxide vapor from sub cooler 126 returns to the ullage volume of vessel 11 through line 133 and line 22 responding to level control 134 making certain the freon sub cooler 126 functions properly other arrangements are possible but not shown line 32 brings liquid carbon dioxide 12 a from tank 24 to.