They allow airflow to pass in a single direction and to forestall it in the other route. They are really built from corrosion resistant extruded aluminum. Will not right away open up entirely upon reaching their start-open up tension. HPR series dampers are flange mounted with counterbalance weights mounted externally https://heavydutystainlesssteelda89011.blogdeazar.com/37077649/dampers-for-industrial-sector-options