insert 25 which is disposed in the space 23 provided by the outer pipe 21 the outer pipe 21 may be formed from stainless steel copper or other metallic material at least one spacer 27 and depending upon the length of the apparatus and the bends therein a plurality of said spacers may be used to provide structural support and spatial arrangement between and among the central pipe 12 the tube member 20 and the outer pipe 21 the spacer s 27 can be welded into position as shown in fig1 one end 24 of the central pipe 12 is connected to a source not shown of high pressure sub cooled liquid nitrogen at approximately greater than 3 barg another end 26 of the central pipe 12 extends through the end cap 30 the first insert 18 does not consume the entire interior space 22 of the tube 20 that is end caps 28 30 seal opposed ends of the tube 20 but provide an entry space 32 and a return space 34 respectively at opposed ends of the tube member 20 a pipe 35 introduces a low pressure liquid nitrogen 36 for sub cooling and circulation at less than approximately 1 barg to the entry space 32 the liquid nitrogen flows in the passageway 16 along the length of the tube member 20 whereupon it reaches the return space 34 before the end cap 30 at which point the flow turns and proceeds along the passageway 17 the.