The walls of a furnace are made of 1.6-ft-thick concrete (k = 0.64 btu/h⋅ft⋅°f and α = 0.023 ft2/h). initially, the furnace and the surrounding air are in thermal equilibrium at 70°f. the furnace is then fired, and the inner surfaces of the furnace are subjected to hot gases at 1800°f with a very large heat transfer coefficient. determine how long it will take for the temperature of the outer surface of the furnace walls to rise to 70.1°f.

Athin walled concentric tube exchanger is used to cool engine oil from 160°c to 60°c with water that is available at 25°c acting as a coolant. the oil and water flow rates are each at 2 kg/s, and the diameter of the inner tube is 0.5 m and the corresponding value of the overall heat transfer coefficient is 250 w/m2. oc. how long must the heat exchanger be to accomplish the desired cooling? cpwater=4.187 kj/kg-candcpengine el=2.035 kj/kg·°c, oil . 120]