Process Heaters www.tempco.com | (800) 323-6859 | email: [email protected] 11-3 Typical Applications Copper Sheath—Process water, water with very weak chemical solutions, potable water, hot water storage for washrooms, showers, cleaning and rinsing parts, for freeze protection of cooling towers and sprinkler systems and other aqueous solutions not corrosive to copper sheath. Sheath temperatures to 350°F (177°C). Incoloy® Sheath—Weak chemical solutions, oils, tar, caustic soda, detergent, alkaline solutions, molten salts, demineralized, deionized or pure water (sheath passivation is recommended), and other aqueous solutions not corrosive to Incoloy® sheath. Air, gas mixtures and superheated steam. Sheath temperatures to 1600°F (871°C). Steel Sheath—Fluid heat transfer media, tar, high to low viscosity petroleum oils, asphalt, wax, paraffin, degreasing solvents, alcohol, molten salt, and other solutions not corrosive to steel sheath. Sheath temperatures to 750°F (399°C). 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. Select the Element Sheath Material 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. Element Watt Density is the wattage dissipated per square inch of the element sheath surface and is calculated with the following formula. element wattage Watt Density = π × element dia. × element heated length Checklist — Selecting the Proper Screw Plug Heater Determine a Safe and Efficient Element Watt Density For a particular application, element watt density will govern element sheath temperature. Factors to consider when choosing a suitable watt density are: 1. Many materials are heat sensitive and can decompose or be damaged if the element is running too hot. 2. Air and other gases that are poor conductors of heat require watt densities matched to the velocity of the gas flow to pre- vent element overheating. 3. When heating hard water and cleaning solutions mineral deposits can build up on the element sheath, acting as a heat insulator and raising the internal element temperature. If these deposits cannot be periodically removed, use a lower watt density element to increase heater life expectancy. 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. Surface Treatments for Stainless Steel and Incoloy ® Elements and other Wetted Parts to Improve Corrosion Resistance Screw Plug Immersion Heater surfaces in contact with the material being heated can be passivated or electro-polished to improve their resistance to corrosion.
