What do you understand by the lumped capacity? What are the physical assumptions necessary for a lumped capacity unsteady state analysis to apply?
Determine time required in minutes for a 50 mm diameter steel sphere (r = 7800 kg/m3, c = 0.46 kJ/kg-K, k = 35 W/m-K) to cool from 600°C temperature to 100°C temperature if exposed to a cooling air at 30°C. The convection heat transfer coefficient is 10W/m-K.