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3-142 Consider a house with a flat roof whose outer dimen- sions are 12 m X 12 m. The outer walls of the house are 6 m high. The walls and the roof of the house are made of 20-cm- thick concrete (k = 0.75 W/m-K). The temperatures of the inner and outer surfaces of the house are 15°C and 3°C, respectively. Accounting for the effects of the edges of adjoin- ing surfaces, determine the rate of heat loss from the house through its walls and the roof. What is the error involved in ig- noring the effects of the edges and corners and treating the roof as a 12m X 12 msurface and the walls as 6 m X 12 m surfaces for simplicity? 3-143 Consider a 25-m-long thick-walled concrete duct (k = 0.75 W/m-K) of square cross section. The outer dimensions of the duct are 20 cm X 20 cm, and the thickness of the duct wall is 2 cm. If the inner and outer surfaces of the duct are at 100°C and 30°C, respectively, determine the rate of heat transfer through the walls of the duct. Answer: 47.1 kW FIGURE P3-143 3-144 A 3-m-diameter spherical tank containing some ra- dioactive material is buried in the ground (k = 1.4 W/m-K). The distance between the top surface of the tank and the ground surface is 4 m. If the surface temperatures of the tank and the ground are 140°C and 15°C, respectively, determine the rate of heat transfer from the tank. 
