According to the air conditioning air system plan of the third floor of the sorting center, the bottom elevation of the bag air duct installation is 3650mm. As the siphon and fire pipe on site have been installed, the bottom elevation of the siphon and fire pipe is 4000mm. The bag air duct of air conditioning can only be installed under the siphon and fire pipe. At the same time, in consideration of avoiding the angle steel L50 bracket of fire pipe and the fluctuation of air conditioning bag air duct during operation, the top elevation of air conditioning bag air duct is controlled at 3900mm, and the diameter of air conditioning bag air duct is 500mm, so the bottom elevation of air conditioning bag air duct is changed to 3400mm