Fans have a long history. China has manufactured simple wooden Kariya windmills for many years before BC, and its principle of action is basically the same as that of modern centrifugal fans . In 1862, the centrifugal fan invented by British Guell Bell, its impeller and casing are concentric, the casing is made of brick, the wooden impeller uses backward straight blades, and the efficiency is only about 40%. It is mainly used for mine ventilation. In 1880, people designed volute casings for exhaust air supply in mines, and centrifugal fans with backward curved blades. The structure was relatively complete.