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 copper fins 14 15 coact to provide the separate passageways 16 17 the flow 43 of the liquid cryogen continues back toward where it was introduced at the entry space 32 to be recirculated again back through the passageway 16 the flow 43 keeps the temperature of the cryogen liquid in the central pipe 12 as low as possible to prevent vaporization of the liquid a circulation pump 40 and a vacuum pump 42 are in communication with the entry space 32 through a line 46 or conduit to circulate the low pressure lin 36 along the fins 14 15 a pressure control unit 44 includes relief valves and a pressure gauge and is disposed for communication with the line 46 for the circulating and vacuum pumps 40 42 respectively although the pressure control unit 44 in fig3 is shown used with the line 46 where the low pressure liquid nitrogen 36 is introduced the pressure control can also be mounted for use at the return space.