A.Construction 1. AL series of centrifugal FDF and IDF have such two structural types as single-suction double-support type and double-suction double-support type. Damper is adopted in the fan outlet for controlling the wind volume. During running, the on-off of the damper is controlled and operated by electric actuator. 2. Acoustic insulation structure is installed to the casing, with which the pneumatic noise can be ensured under 85 dB(A) according to the requirement of customers. Having been taken consideration of casing deformation caused by operating in thermal state, optimum matched clearance is ensured for all parts, to make the stable aerodynamic and mechanical performance reliable. 3. In order to ensure enough rigidity, integral forging is applied to the main shaft, i.e. the main shaft and the impeller hub disk constitute an unity. Meanwhile, for the main shaft and the motor, direct-connection drive of gearing coupling is selected, which results in many advantages, such as convenience in repairing or replacing the impeller, reducing the rotor weight and lengthening the bearing service life. 4. Considering the dust that contained in the conveying gas, reasonable wear-resisting treatment has been made to relevant part of the impeller. 5. Outer spherical sliding bearing is used for the fan, with water cooling system inside, so that the operating temperature can be kept under 75℃. B.Characteristics 1. With flat performance curve and wide region of high efficiency, the highest efficiency can be 85% or above when the damper opens completely. (refer to the performance table) 2. Easy to maintenance and overhaul. Split casing is convenient for the transportation and assembly. Manhole is opened in three different directions, so that the important parts can be easily maintained and overhauled after the fan stop working. 3. The stiffener plate is reasonably arranged to the outer surface of the casing, which has good rigidity. It's convenient for customers to control the site noise according to their own requirement, i.e. sound and heat insulation treatment. 4. The outer spherical sliding bearing has fine centering character. With immersion oil natural lubricating and no any other lubricant power system, the bearing damage and oil