A spherical tank (k = 15 W/m ∙ K) with an inner diameter of 3 m and a wall thickness of 10 mm is used for storing hot liquid. The hot liquid inside the tank causes the inner surface temperature to be as high as 100°C. To prevent thermal burns on the skin of the people working near the vicinity of the tank, the tank is covered with a 7-cmthick layer of insulation (k = 0.15 W/m ∙ K) and the outer