The calculation of the size of the air duct is based on the arrangement of the system and equipment, the material of the air duct, the position of each air supply and exhaust point and the air volume, reasonable layout of air duct for hydraulic calculation sketch. In the draft calculation, pipe section number, and mark the length of pipe section and air flow, pipe section length is generally calculated according to the length of the center line of two pipe fittings, do not deduct the length of pipe fittings such as tee, elbow itself. The most adverse loop of the system is selected, usually the loop with the greatest distance or local resistance. Select the appropriate air flow rate, as listed in the previous section of the recommended flow rate and maximum flow rate meter. According to the given air volume and the selected flow velocity, the end face dimension of the pipeline is calculated section by section, and it conforms to the uniform specification of the rectangular duct. Then, the actual flow velocity in the duct is calculated according to the selected section size and air volume. The air flow through the rectangular duct g can be calculated as follows: