an exchanger a bypass passage constricted at its upper end providing a flow path in parallel with flow through an exchanger from a feed junction to a more elevated return junction the flow being conducted in an upwards direction to the feed junction and from the feed junction into the bypass passage the apparatus providing closed loop circulation of cryogenic liquid to a heat exchanger with the driving force for the circulation being supplied by the differential head between cryogenic liquid in the feed conduit and spent fluid in the return conduit application_number US 49983374 A description delivery apparatus according to the invention and designated generally by numeral 10 includes separator vessel 12 enclosed in vacuum jacket 14 and supplied by cryogenic fluid which is typically liquid nitrogen through conduit 16 emptying into vessel 12 through automatically controlled valve 18 cryogenic liquid 20 is maintained in the lower portion 22 of vessel 12 and the vapor therefrom occupies upper portion 32 of vessel 12 and is vented through vent 24 feed conduit 26 at its upper end 27 draws liquid from separator vessel 12 and connects at its lower end 29 to delivery fitting 28 return conduit 30 shaped as an annular passage coaxial with feed conduit 26 connects at its upper end 31 with vapor space 32 above fluid 20 and at its lower end 33 to lower fitting 28.