interior of one end 30 of a spiralled tube 32 the spiralled tube 32 is formed as a helix of tubing having a rectangular and specifically square cross section as can be seen from fig1 the outer diameter of the spiralled tube 32 is somewhat less than the inner diameter of the housing exterior 12 at the same time the inner diameter of the spiralled tube 32 is considerably greater than the diameter of the inlet 20 the spiralled tube 32 forms part of a heat exchanger wherein heat exchanged between the higher density components of the inlet stream and lower density components of the inlet stream occurs in the preferred environment of use namely spacecraft operation the higher density components are employed to absorb heat from the lesser density components after passing through the heat exchanger including the spiralled tube 32 those components that were originally of relatively high density may be vented to space via an outlet 34 at the end 16 of the heat exchanger 10 the outlet 34 being disposed centrally of the housing exterior 12 those components that were originally of lesser density and are contained within the tube 32 exit via a line shown schematically at 36 in a space environment cryogenic liquid propellant is stored in a reservoir 38 including a vent control system 40 therefor a low power pump 42 is utilized to circulate the fluid when the vent control 40 requires such circulation as for example when there is a pressure buildup consequently in.