Aerovac® “AVRD 33” model series Round Volume Control Dampers (VCD) are designed to regulate air volume in circular ducts. These dampers are easily accessible with round ducts. The casing and blades are made of 22 gauge galvanized steel sheets The blade is mounted on nylon bushes and is manually operated with a quadrant set. The movement of the blade can be adjusted from “full open” to “complete shut off position.”