Chilled air at 15?C enters a horizontal duct at a speed of 1 m/s. The duct is made of thin galvanized steel and is not insulated. A 30 m section of the duct runs outdoors through humid air at 30?C. Condensation of moisture on the outside of the duct is undesirable, but it will occur if the duct wall is at or below the dew point temperature of 20?C. For this problem, assume that condensation rates are so low that their thermal effects can be ignored.
a. Suppose that the duct's square cross-section is 0.3 m by 0.3 m and the effective outside heat transfer coefficient is 5 W/m2K in still air. Determine whether condensation occurs.
b. The single duct is replaced by four circular horizontal ducts, each 0.17 m in diameter. The ducts are parallel to one another in a vertical plane with a center-to-center separation of 0.5 m. Each duct is wrapped with a layer of fiberglass insulation 6 cm thick (ki = 0.04 W/m·K) and carries air at the same inlet temperature and speed as before. If a 15 m/s wind blows perpendicular to the plane of the circular ducts, find the bulk temperature of the air exiting the ducts