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Waste heat from the stack gas (800K) of an incinerator is to be used as the heat source for electricity generation. To do so, 2000m3s1 of the hot stack gas is diverted to the shell side of a large heat exchanger to heat 400kgs1 of engine oil running through 40 thin-walled tubes (diameter 2cm ) with 2 passes and 7.5m per pass. The engine oil enters the tubes at 350K. The properties of the stack gas can be well-estimated using air. (a) By using the NTU method, calculate the heat duty of the shell-tube heat exchanger as well as the outlet temperatures of the stack gas and engine oil. Assume well-developed flow within the tube. Hint: For initial guess, you may assume the outlet temperature of the engine oil to be 470K and the outlet of stack gas to be 700K, but do not forget to verify at the end of your calculations. (12 markah/marks) (b) Briefly explain why the heat exchanger is under-designed. Give your three best suggestions on how you would further improve the heat duty of the heat exchanger. (4 markah/marks) c) A small section of the insulation layer on the gas inlet (a circular steel pipe, 0.21 m outer diameter) to the shell side accidentally came off. The shell surface is now exposed to a constant breeze at (h=20Wm2K1) and the surrounding, both at 300K. Sketch the equivalent thermal circuit and calculate the temperature of the exposed surface given that the steel pipe is 0.01m thick while the shell has a thermal conductivity of 14Wm1K1 and emissivity of 0.85