A 1 kW commercial electric heating rod, 8 mm in diameter and 0

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A 1 kW commercial electric heating rod, 8 mm in diameter and 0.3 m long, is to be used in a highly corrosive gaseous environment. Therefore, it must be covered by a cylindrical sheath of fireclay. The gas flows by at 120?C, and h is 230 W/m2K outside the sheath. The surface of the heating rod cannot exceed 800?C. Determine the maximum allowable sheath thickness and find the outer tem- perature of the fireclay. (Hint: Use the heat flow and temperature boundary conditions with thermal resistances in series.)