Folded Box Type Pre Air Filter Low Efficient Filter With Aluminum Alloy Frame Product Description Filter material folding process: Fold the flat filter material into a "wavy" or "accordion" shape. By increasing the unfolded area of the filter material (up to 3 to 5 times that of the flat type), the filtration efficiency and dust holding capacity can be enhanced within a limited space. This pre-filter is constructed with a durable polyester filter cotton, providing excellent