Abstract: A rotary drum filter having a reduced amount of fluff in the enclosure during operation. The volume of the enclosure and the mass flow rate of fluff coming into the enclosure are taken into account when determining a proper speed for the rotary drum so that fluff build up on the filter medium is reduced. By properly balancing these three items, fluff may be removed from the filter enclosure at a rate which is equal to or greater than the rate at which the fluff enters the enclosure. The structure of the rotary drum enclosure provides a high velocity, smooth air flow at the bottom of the filter enclosure. The incoming air and fluff are distributed over the entire length of the floor, so that turbulence is created along the length of the floor. In addition, the floor is curved so that there are no joining positions along the floor where dust can accumulate, and so all fluff entering the enclosure is blown to the top of the enclosure.