The circular duct fan (also known as UFO fan and coaxial centrifugal fan) is mainly composed of four parts: shell, impeller, external rotor motor and junction box. The shell is made of high quality cold rolled iron sheet that can be deep drawn and is drawn by a die, with strong corrosion resistance. The pipe design optimized by computer enables the impeller to produce maximum efficiency. The air inlet and outlet have the same diameter and size, and are designed according to the standard pipe, which is convenient for installation, maintenance and cleaning. Specifications are Φ 100mm; Φ 125mm; Φ 150mm; Φ 160mm; Φ 200mm; Φ 250mm; Φ 315mm。 The impeller blade is of backward curved design, and the impeller and shell are well matched as a whole, so that the fan can produce the maximum air volume and efficiency. The motor is a single-phase asynchronous motor with an external rotor. The rotor of the motor is outside, and the impeller is directly pressed with the motor rotor. The impeller is directly driven by the motor rotor, which can greatly reduce the volume of the fan, and has stable operation, high efficiency and long service life. The junction box is made of PVC material with flame-retardant fiber by injection molding. It has good fire resistance and is safe to use. It is easy to assemble and disassemble the wiring outside the casing.