Secondary surface (fins) are of the plate-fin design using aluminum or copper, with die-formed collars. The fin design is to be flat, waffle, or sine-wave in a staggered tube pattern to meet performance requirements.
Collars will hold fin spacing at a specified density and cover the entire tube surface. Aluminum properties are to be alloy 1100 per ASTM B209, with 0 (soft) temper; copper is to be alloy 11000 per ASTM B125-06 with soft (anneal) temper. Fins are to be free of oils and oxidation.