Closed damper doors, also known as cutoff doors, air duct dampers, or flue dampers, are commonly used in power plants and other industrial settings as a means of controlling the flow of smoke or air through ducts. They have a 90-degree opening and closing mechanism, providing full open and full close functions to allow for complete flow or closure of a system's pipeline medium. The closed damper doors consist of several parts, including the structure, blades, sealing strips, main shaft, bearings, shaft seal, crank and connecting rod mechanism, and actuator (optional). The structure is made of steel plate molding, steel beam welding, or casting, while the blades are formed by steel plate molding or casting. The streamlined blade cross-section ensures minimal pressure loss. Special elastic sealing structures are installed between the blades and the structure to ensure a sealing rate of over 99% when closed, with special orders reaching over 99.5%. The blades are connected to the main shaft with special fasteners. After installation on the pipeline, any component can be easily removed, and the system can be conveniently maintained or replaced while in operation. The shaft neck features a sealing structure and bearing, with the sealing structure providing excellent sealing performance, no leakage, and high-temperature resistance. The bearings have self-aligning and self-compensating functions, automatically compensating for the axial alignment errors of the front and rear shafts, ensuring minimal resistance, no sticking, and smooth rolling of the closed damper doors. The crank and connecting rod mechanism is equipped with bearings to ensure minimal transmission resistance. For large-sized closed damper doors, a double-armed heavy-duty crank and connecting rod mechanism is used to ensure precise opening and closing actions. Manual, electric, or pneumatic actuators can be directly connected to the drive shaft or connected remotely through the crank and connecting rod mechanism. Actuators can also be supplied by the user.