Oxidation, Corrosion, and Temperature Considerations. On extended surface (finned or studded) tubes, the fin or stud material is chosen largely on the basis of oxidation resistance. Because fin temperature will often approach flue gas temperature, higher alloys are usually required for fins than for tubes. Fins are typically thin and cannot tolerate much corrosion. Also, buildup of an oxide film on the fins will impede heat transfer. For these reasons, maximum allowable fin temperatures are chosen so as to assure negligible oxidation rates. API-560, Table 2, gives temperature limits for various fin materials.
Thermal Expansion. Do not use chrome-nickel stainless fins on carbon or chromemoly steel tubes; and don’t use carbon or chrome-moly fins on chrome-nickel tubes. Expansion coefficients should be similar; preferably within 20% of each other. Chrome-nickel austenitic stainless steels (300 series) have a coefficient of expansion nearly 50% higher than carbon and chrome moly steels (including 400 series stainlesses). These limitations are noted in HTR-MS-1350.