An apparatus for the separation or liquefaction of a gas at cryogenic temperatures which comprises an isolated chamber comprises at least one front distillation column operating at cryogenic temperatures and also a pipe for transferring fluid coming from or going to the column, the pipe having a diameter D comprising a bend for changing the direction of flow of the fluid, with profiled deflector vanes placed inside the bend, with their concavity towards the centre of the bend forming a plurality of ducts.