18 india Nrm Opn Rnd Stel Zone Cntrl Dampr For Nominal Duct Diameter 18 in Damper Application Motorized Number of Panels Not Applicable Overall Height 20 1/8 in Overall Length 18 1/2 in Rough Opening Shape Round For Duct Shape Round Overall Width 18 in Frame Material Thickness Not Applicable Blade Material Galvanized Steel Maximum Air Pressure Not Applicable Overall Diameter 18 in Rough Opening Length 18 in Rough Opening Width 18 in Damper Type Dynamic