Hot combustion gases of a furnace are flowing througha concrete chimney (k =1.4 W/m · °C) of rectangular cross section. The flow section of the chimney is 20 cm ×40 cm, and the thickness of the wall is 10 cm. The average temperatureof the hot gases in the chimney is Ti=280°C, and the averageconvection heat transfer coefficient inside the chimney is hi=75 W/m2· °C. The chimney is losing heat from its outer surfaceto the ambient air at To=15°C by convection with a