Boiler tubes exposed to the products of coal combustionin

Chapter 13, Problem 13.98

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Boiler tubes exposed to the products of coal combustionin a power plant are subject to fouling by the ash (min-eral) content of the combustion gas. The ash forms asolid deposit on the tube outer surface, which reducesheat transfer to a pressurized water/steam mixture flow-ing through the tubes. Consider a thin-walled boiler tube(Dt?0.05 m) whose surface is maintained at Tt?600 Kby the boiling process. Combustion gases flowing overthe tube at T??1800 K provide a convection coefficientof , while radiation from the gas andboiler walls to the tube may be approximated as thatoriginating from large surroundings at Tsur?1500 K. a) If the tube surface is diffuse and gray, with?t?0.8, and there is noash deposit layer, whatis the rate of heat transfer per unit length, q?, tothe boiler tube?(b) If a deposit layer of diameter Dd?0.06 m and thermal conductivity k?1W/m?K forms onthe tube, what is the deposit surface temperature,Td? The deposit is diffuse and gray, with ?d?0.9,and Tt, T?, , and Tsurremain unchanged. What is the net rate of heat transfer per unit length, q?,to the boiler tube?(c) Explore the effect of variations in Ddand onq?, as well as on relative contributions of con-vection and radiation to the net heat transferrate. Represent your results graphically.