To reduce heat losses, a horizontal rectangular duct that is W = 0.80 m wide and H = 0.3 m high is encased in a metal radiation shield. The duct wall and shield are separated by an air gap of thickness t = 0.06 m. For a duct wall temperature of Td = 40°C and a shield temperature of Tsh = 20°C, determine the convection heat loss per unit length from the duct.