All dampers are square frame, 4 3/4" deep and have 7/8" top flange. These all-aluminum, self-acting back draft dampers work on a pneumatic principle opening only when fans are in operation, and automatically closing when fans are turned off. Aluminum, multi-leaf, interconnected blades with stainless steel sleeve bearings are set into square frames designed to be mounted inside of curb or framed in opening between the exhaust fan and the ductwork. Dampers are not designed for use with up-blast units that exhaust grease-laden vapors.