Apparatus for the vaporization of a heavy hydrocarbon feedstock with steam includes a frusto-conically-shaped element which is capable of inducing a gentle swirl pattern to a liquid-containing stream, which device is connected to the feed inlet pipe of the mixing nozzle. This swirl-inducing device effects a 90° bending of the flow direction of the hydrocarbon feedstock while simultaneously effecting a swirl pattern of the liquid part thereof, thereby forcing the liquid against the wall of the feed inlet pipe extending downwardly to the mixing nozzle with such velocity component perpendicular to the tube's longitudinal axis that an annular flow pattern is re-established.