Cold conditioned air at 12°C is flowing inside a 1.5-cm-thick square aluminum (k = 237 W/m·K) duct of inner cross section 22 cm × 22 cm at a mass flow rate of 0.8 kg/s. The duct is exposed to air at 33°C with a combined convection-radiation heat transfer coefficient of 13 W/m²·K. The convection heat transfer coefficient at the inner surface is 75 W/m²·K. If the air temperature in the duct should not increase by more than 1°C determine the maximum length of the duct.