Subclass 1/18: Tubular elements; Assemblies of tubular elements (specially adapted for movement F28F 5/00) Tubular elements or assemblies thereof with means for increasing heat-transfer area, e.g. with fins, with projections, with recesses (crimped or corrugated elements F28F 1/06, F28F 1/08) the means being only outside the tubular element and extending longitudinally (F28F 1/38 takes precedence) the means being integral with the element, e.g. formed by extrusion (F28F 1/22 takes precedence) the element being built-up from finned sections