PURPOSE: To make the development of the drift of heat transfer medium hard and have the wide ensured heating surface area and high high-temperature strength by a structure wherein a tubular cover is put onto the peripheral surface of a vessel and recessed grooves beads protruding towards the interior of the vessel are formed on said peripheral surface of the vessel covered by said tubular cover.

CONSTITUTION: Protruded stripes 2 and 2, which are protruded outwards, are provided parallel to each other near the lower end and near the central part on the peripheral surface of a tubular vessel 1. In addition, three recessed grooves 3, 3 and 3, which are protruded inwards, are arranged between said protruded stripes 2 and 2. A tubular cover 4 covers the space between the two protruded striped 2 and 2 in such a manner that the cover 4 spans the two protruded stripes 2 and 2 in the vessel 1 as mentioned above. A gap equipped with the recessed grooves 3, 3 and 3 is formed between the peripheral surface of the vessel 1 covered with the cover 4 and the cover 4 acts as a flow passage 5 for heat transfer medium (not shown). When heat transfer medium flows in the flow passage 5 as indicated with a solid line with an arrowhead, the natural convection collides against the side surfaces of the recessed grooves 3, 3 and 3 facing to the flow passage 5 within the vessel 1, resulting in agitating the convection as indicated with chain lines with arrowhead.