... Tubes Bare Tubes Materials:Bare tubes can be of any material such as brass, cupronickel, carbon steel, ... muff over a liner tube. The process of extrusion hardens the fins and prevents dissimilar metal ... , nor do they uncoil, and the lining does not come loose, thereby maintaining the heat exchanger ... or copper.Bare Tubes Materials:Bare tubes can be of any material such as brass, cupronickel, carbon steel, ...