The pressure loss varies with the shape and size of the pipe. With the same cross-sectional area, the circular pipe saves the most materials, the pressure loss is the smallest, the stress is uniform, it is not easy to deform, and it is not easy to produce noise. The second is the square pipe, and the third is the rectangular pipe. However, the manufacture of round pipe fittings is relatively complex, and the connection and sealing installation is not as convenient and reliable as square pipe. It occupies a large indoor height which is limited by the height of installation space. Therefore, circular pipes are generally not used indoors and are mainly used outdoors. The flatter the rectangular tube with the same cross-sectional area, the more plates used, the greater the pressure loss. The eddy current generated at the four corners increases the pressure loss, which is the main cause of noise. The area of one side of the rectangular pipe is relatively large, and it is easy to deform under pressure and produce sound. It is necessary to thicken the plate and adopt the strengthening process. Therefore, if space permits, try to make the four sides of the rectangular pipe equal.