When it is necessary to prevent contamination of air or gas passing through the fins by liquid escaping from any leaks in the tubes - as for the cooling of electric machines - the double-tube construction is the solution that guarantees improved safety. Every tube of the exchange bundle is composed of two coaxial tubes, between which canals are obtained that convey any leakage towards collection chambers placed close to the tube sheets, where it can be detected without it escaping and thereby damaging other equipments.