Single Blade Dampers
http://mmma.org.uk/launch-nukon-fiber-laser-cutting-machines-uk Single Blade Dampers can be built to fit any shape or size air system and are ideal for low leakage and shut off applications. They are a cost-effective solution for in-duct shut off applications and ideal for paint laden airstreams and low leakage (moderate to high temperature) applications.
http://thesightseer.org/audio/The Wall Street Journal - TUE 04-JUL 2.mp3 Butterfly Dampers, or Butterfly Backdraft Dampers, can be built to customer specifications are ideal for shut off and backdraft prevention applications when system is not in use. Air Systems has created butterfly backdraft shut off dampers for a variety of customers to satisfy their unique criteria. Seal designs include: Swing-Through, Tadpole, and Step-Seat.