21 In.H-W 5 1/4 In.L SS Backdraft Dampr Damper Application Backdraft For Nominal Duct Diameter Not Applicable Frame Material Stainless Steel Number of Panels 1 Maximum Air Velocity 2500 fpm Overall Height 21 in Rough Opening Diameter 18 1/2 in Rough Opening Shape Square Fits Fan Diameter 18 in For Duct Shape Not Applicable Free Area 1.9 sq ft Overall Width 21 in Frame Material Thickness 0 in Overall Length 5 1/4 in Connection Type Not Applicable Frame Depth 2 in