The invention relates to a system consisting of a hermetically-sealed box, inside which a bundle of parallel tubes are disposed, wherein said tubes enter and exit the box through respective tanks attached to the rear wall of the box, and the ends of the tubes have U or similar parts that revert the flow 180 sexagesimal degrees, the opposite direction branch extending a certain length until same connects with the corresponding tank, which is placed far from the end in question and near the central transverse line on the rear wall of the box.