Search for question
Question

Problem 2: Boiler tubes exposed to the products of coal combustion in a power plant are subject to fouling by the ash (mineral) content of the combustion gas. The ash forms a solid deposit on the tube outer surface, which reduces heat transfer to a pressurized water/steam mixture flowing through the tubes.Consider a thin-walled boiler tube (D=0.05 m) whose surface is maintained at T=600 K by the boiling process. Combustion gases flow over the tube at T=1800K and an average convection coefficient of h=150W/m?-K, while radiation from the gas and boiler walls to the tube may be approximated as that originating from large surroundings at Tsurr=1500 K. (a) If the tube surface is diffuse and gray, with Et = 0.6, and there is no ash deposit layer, what is the rate of heat transfer per unit length (in kW/m) to the boiler tube? Ans: 54.6 kW/m (b) If a deposit layer of thickness t=0.007 m and thermal conductivity k=1 W/m-K forms on the tube,what is the deposit surface temperature, Ta? The deposit is diffuse and gray, with ɛa= 0.9,and T, To, and Tsurr remain unchanged. Hint: this is 1D steady conduction in the deposit, but the surface-of the deposit has both convection and radiation. You may need to have an iterative solution. Ans: 1425 K(c) What is the net rate of heat transfer per unit length to the boiler tube for the case with the deposit?Do you think this would be a problem for boiler operation? Why?

Fig: 1

Fig: 2

Fig: 3

Fig: 4

Fig: 5