Portions of fluid components are mixed during transport through a tube by continuously disturbing their flow pattern in a controlled manner during their passage through the tube. The disturbance is created by providing the tube with a radially inward profile which in a preferred form is an internal screw-thread of a thickness 0.25 to 0.75 times the internal diameter of the tube and having a pitch 0.75 to 1.5 times the internal diameter of the tube.
