They allow airflow to pass in a single course and to stop it in the opposite direction. They're constructed from corrosion resistant extruded aluminum. Product 0540V Broad Body Static Curtain Fire Damper is UL permitted for use the place setting up codes demand the defense of HVAC ductwork penetrations in https://dampers-for-industrial-se45677.blog4youth.com/37662180/a-secret-weapon-for-dampers-for-industrial-sector