This invention relates to a coil comprising a first helical tube segment (511) and a second helical tube segment (512, 513) extending, parallel to the first helical segment (511), between a distributor (53) and a manifold (54), said first and second helical segments (511, 512, 513) being centred on a common geometrical axis (X.sub.5), with substantially the same bending radius (R.sub.1) and nested, such that they form together a globally cylindrical bundle (51). The method comprises a step which consists in interleaving the helical tube segments (511-513) so as to form a globally cylindrical bundle. The invention also concerns a reactor equipped with such a coil which can be used for treating a viscous medium or for carrying out chemical reactions, such as 6-6 polyamide polymerization.