·BFU adopts the clean and safety theory of modern large area new clean room, inhales air from the top, and then filters the air out. BFU is ceiling type air supply. It is often used as a purification unit in the relatively low purification level purification workshop, because its air outlet speed is larger. Under the condition that the air inlet volume of the low level workshop is smaller, it is easier than FFU to make the workshop mixed flow and reduce the self-cleaning time.