QUESTION 3
a) In a coal-fired power plant, a furnace wall consists of a 125-mm-wide refractory brick and a 125-mm-wide insulating firebrick separated by an air gap. The outside wall is covered with a 12mm thickness of plaster. As given in Figure 1, the inner surface of the wall is at 1100°C, and the room temperature is 25°C. The heat-transfer coefficient from the outside wall surface to the air in the room is 17 W/mK, and the resistance to heat flow of the air gap is 0.16 K/W. The thermal conductivities of the refractory brick, the insulating firebrick, and the plaster are 1.6, 0.3, and 0.14 W/mK respectively. Using thermal resistances, calculate:
i. The rate of heat loss per unit area of wall surface [6 Marks]
ii. The temperature at each interface throughout the wall. [4 Marks]
iii. The temperature at the outside surface of the wall. [3 Marks]
b) In an air-conditioning plant, fluid at 20°C flows through a duct which has a wall thickness of 100 mm. The surrounding temperature is 10°C. The convective heat-transfer coefficient inside the duct is 12 W/mK, and that of outside is 5 W/mK. The thermal conductivity of the duct is 0.5 W/mK, and the external emissivity is 0.85. Assuming negligible internal radiation, determine the heat flow rate through the duct per square meter of duct surface and external wall surface temperature. Use an initial trial value of 40°C for the external duct wall temperature [12 Marks]
