Paper Frame Filter The pre-filter is a low-efficiency filter used in HVAC systems that functions to filter out large sizes of dirt and dust in the air. This filter is usually placed at the very beginning of the AHU filter system right before the Medium filter or Pocket Filter because it has a large dust capacity and good durability at an economical price. In general, the Pre-Filter has a frame made of aluminum or cardboard with a mesh separator and galvanized wire which serves to increase the surface area.