Here Sectional details of duct .including 25mm overlap to all side of damper , bolt and screw tank weld or steel pop reverts 300mm ,clearance is 6mm, The ventilation system in a building consists of air moving devices such as fans and blowers and a network of ducts to exhaust the contaminated indoor air and to bring in air from the outside of the building. Ducts are channels, tubes, or pipes that carry moving air. For more details download this file .