Prevent drafts with our Plastic Dryer Vent Draft Blocker. When your clothes dryer is in use, the exhaust opens the built-in damper. Gravity keeps it closed when you’re not drying laundry. Plastic Is Very Durable. Damper unit comes fully assembled, and installs to the clothes dryer duct and vent pipe. Great for kitchen hood fan. PLASTIC RECTANGULAR AND ROUND SYSTEM Plastic air ducts are designed for use in small living spaces and of ce premises, where the ventilation system is most often locted under a suspended ceiling. Plastic rectangular or round system allows easy installation of small air lines with different turning angles for the horizontal and vertical planes, due to the available convenient auxiliary connections and adapters.