321 Heater Tubes

Process streams are often heated by immersion heaters. Engineers are evaluating CFCC materials in immersion heaters to melt aluminum. Aluminum is normally melted in reverberatory furnaces or in furnaces with radiant burners. These methods have several limitations: (1) efficiency is limited, resulting in at least 60% of the heat going up the stack; (2) vapor-phase reactions in the space between the tubes, furnace ceiling, and melt result in the formation of oxide scale that contaminates and lessens the quality of the aluminum; and (3) heating is nonuniform. All affect aluminum quality and cost.

An alternative method is gas-fired immersion tube heaters, with monolithic silicon carbide ceramic tubes immersed directly into the aluminum. These tubes have demonstrated increased efficiency, more uniform heating, and substantially reduced contamination. However, they are susceptible to thermal shock.

FIGURE 3.5 CFCCs can be made in wide variety of shapes and

sizes.

Engineers are evaluating tough CFCC materials because they do not have these limitations. Also, CFCC materials are not wetted by molten aluminum and are not chemically attacked by either the aluminum on the outside of the tube nor the combustion gases or heating element on the inside (Fig. 3.6).

Textron Systems Corporation, Lowell, Massachusetts, is using a computer-controlled 5-axis winding machine with a seamless mandrel to fabricate and test these immersion tubes. Textron is teamed with Deltamation (a furnace designer), F. W. Shafer (a furnace manufacturer), Doehler-Jarvis (an aluminum caster), and an automotive manufacturer to evaluate their tubes. These tubes have survived 1752 h operating in a production aluminum caster. This exposure included the normal practice of cycling through 30 h of melting at 870°C (1600°F) with 15 min of pour time. The CFCC survived the thermal shock of 2 cold starts and the 50 tube withdrawals during the pours. They continue operating toward a goal of 3000 h. Success would realize benefits of reduced downtime, increased product yield, improved quality, increased efficiency, reduced energy consumption, reduced emissions, and lower operating costs. Energy savings accrue due to an increased heat transfer improvement of 40% and increased product yield. Fifty trillion Btu/year would be saved if CFCC tubes were used to melt all U.S. aluminum. Emissions are reduced because less fuel is consumed and recuperation is enabled.

36-in.-long, 6-in.-diametef CFCC Immersion Tube

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