Stephens has two main models of silo vents the SV 170 which has 16 bags and a 170 sq ft of cloth area, the SV 265 unit which has 25 bags and 250 sq ft of cloth area. These units are designed to filter material back into the silo through a shaking mechanism. All moving parts and bags are replaceable. The Standard bag for the SV 170 and SV 265 are 6’ in length and 7” in dia. Other vents, and bags are available please consult the factory with any questions.