Fan Filter Unit (FFU) is a type of air filtering equipment. It is used to supply purified air to cleanrooms or microenvironments by filtering out harmful airborne particles from recirculating air. FFUs can be used in place of a more conventional recirculating air unit such as a ducted or plenum air system. FFUs are often used because of height restrictions in the space; they are the only systems that work in setups with smaller internal floor dimensions. Additionally when less than 20 filters are involved, a fan powered HEPA, FFU design is generally considered to be less expensive than a more conventional ducted supply system.