A circular sheet metal duct carries refrigerated air to a cold storage room for apples. The duct itself is 200 mm in outer diameter. The duct wall thickness is 2 mm. To reduce the heat gain from the surrounding air, the duct is wrapped with insulation. The flowing air maintains the inner surface of the duct at 0°C. The outer surface temperature of insulation would be maintained at 23°C by the room air. Thermal conductivity of the sheet metal is 60 W/m·K and that of the insulation is 0.03 W/m·K. Assuming the heat transfer to be at steady state, what thickness of insulation should be put on the duct to keep the rate of heat gain by the refrigerated air per meter length of the duct to 30 W?