Process Heaters www.tempco.com | (800) 323-6859 | email: [email protected] 11-27 1. The temperature of the corrodent — As temperature increases the degree of corrosion increases. Also remember that usually the element temperature is higher than the material it is heating. 2. The degree of aeration to which a corrodent is exposed — Stagnant conditions can deprive the stainless steels of oxygen, which is required to maintain their corrosion resistant surface. 3. Velocity of the corrodent — Increased velocity can increase the corrosion rate. Note: See pages 16-12 through 16-20 for the recommended sheath materials for many immersion heating applications. If you are purchasing the material you are heating, check with the supplier for their recommendations. Optional Element Sheath Materials 304 Stainless Steel — A Chromium (18-20%), Nickel (8-11%), Iron Alloy used in the food industry, sterilizing solutions, air heating and many organic and inorganic chemicals. 321 Stainless Steel — A Chromium (17-20%), Nickel (9-13%), Iron Alloy modified with the addition of titanium to prevent carbide precipitation and the resulting intergranular corrosion that can take place in certain mediums when operating in the 800-1200°F (427-649°C) temperature range. Incoloy® 840 — A Nickel (18-20%), Chromium (18-22%), Iron alloy. Incoloy 840® has about 10% less nickel than Incoloy 800. Used in many air heating applications where it has exhibited supe- rior oxidation resistance at less cost than Incoloy 800®. Incoloy® 825 — A Nickel (38-46%), Chromium (19.5-23.5%), Molybdenum (2-3%) Iron alloy. Consult Tempco for more information. Select the Element Sheath Material Standard Element Sheath Materials Incoloy® 800 — A Nickel (30-35%), Chromium (19-23%), Iron alloy. The high nickel content of this alloy contributes to its resistance to scaling and corrosion. Used in air heating (also see Incoloy® 840) and immersion heating of potable water and other liquids that are not corrosive to an Incoloy® 800 sheath. Low Carbon Steel — Applications include fluid heat transfer media, tar, high to low viscosity petroleum oils, asphalt, wax, molten salt, and other solutions not corrosive to a steel sheath. 316 Stainless Steel — A Chromium (16-18%), Nickel (11-14%), Iron Alloy with Molybdenum (2-3%) added to improve corrosion resistance in certain environments, especially those that would tend to cause pitting due to the presence of chlorides. Applications include deionized water. Copper — Mainly used in clean water heating for washrooms, showers, rinse tanks and freeze protection of storage tanks. Sheath Material Selection CORROSION. In addition to selecting a sheath material that is compatible with the heated medium, other factors that affect corrosion need to be considered: Surface Treatments for Stainless Steel and Incoloy ® Elements and other Wetted Parts to Improve Corrosion Resistance Flanged Immersion Heater surfaces in contact with the material being heated can be pas- sivated or electro-polished to improve their resistance to corrosion. Passivation removes surface contamination, usually iron, so that the optimum corrosion resistance of the stainless steel is maintained. Surface contamination would come from the small amount of steel that may be worn off a tool during the manufacturing process. Passivating is accomplished by dipping the heater in a warm solution of nitric acid. Electro-Polishing is an electrochemical process that removes surface imperfections and contaminants, enhancing the corrosion resisting ability of the stainless steels. The resultant surface is clean, smooth and bright. Many medical and food applications require this finish. Optional flange materials include: ✴ 304, 304L Stainless Steel ✴ 316L Stainless Steel ✴ Incoloy® 800 Gaskets of different types, including spiral wound metal with non-metallic filler, are available to properly seal any flanged heater. Gasket material choice depends on operating conditions and fluid compatibility. Consult TEMPCO for help with your selection. Select Optional Flange and Gasket Materials Checklist — Selecting the Proper Flanged Heater, continued
