Helicoidally finned tubes

In a helicoidally finned heat exchange tube, comprising an inner tubular member and an outer helical member, the helical member having substantially circular and solid turns perpendicular to the surface of the tubular member and having ripples which extend inwardly from the outer periphery of the turns and the depths of which decrease radially inwardly, adjacent turns being separated from one another by a substantially continuous gap;

• the improvement in which the helical member (22) has rippled sections (S1, S2) alternating with level sections, the rippled sections subtending an angle not exceeding about 90 degrees and the level sections having a continuous smooth surface, and both types of sections on successive turns registering with one another in the direction of the axis (32) of the tubular member (20) and the level sections on successive turns defining between them gaps confined by walls having smooth surfaces, thereby facilitating a cleaning of said gaps between said smooth surfaces following deposit of solids in said gaps between said smooth surfaces from gas flowing in a direction perpendicular to the bisector of said angle subtended by said rippled sections, the spacing of the sections being substantially equal to a quarter of the circumference of the tubular member so that the rippled sections of the helical member occupy diametrically opposite positions on the tubular member.