Foldable Polyester Fiber Air Pre Filter For Initial Filtration In HVAC System With Increased Filtration Area Product Description A pre-filter is used for the initial filtration at the front end, made of polyester fiber filter cotton. Foldable structure: Adopting a folding design, it significantly increases the effective filtration area of the filter material (compared to flat-type filters, the filtration area can typically be increased by 3-5 times). Under the same air volume