The owners of a large furnace are interested in estimating how long it will take after shut down before their workers can open the furnace and comfortably enter for repairs. A comfortable" inside wall temperature which will allow their people to work is 30C. The furnace is cubical in shape. The inside dimension is 16 m and the wall thickness is 1.4 m. Initially the inside wall temperature is 900C, and the ambient is always at 20C. The average heat transfer coefficient (convection + radiation) on the outside is 28 W/mC. The average properties of the wall are: k = 0.9 W/mC, p = 2400 kg/m, and c = 0.7 kJ/kgC. 16 m 16 m 1.6 m 16 m