Gravity dampers at discharge of two tunnel ventilating fans are only about 50% open noticeably degrading output capacity of fans. (See previous picture’s explanation of importance of fan performance). Fan output is adequate however.

Dampers are dirty and hanging up. The static pressure penalty for these dampers is likely over .5” at design flow* out of the 1.25” total design for the fans.
Recommend repair; or better yet, replacement of the gravity dampers with low loss actuated dampers since they appear to be in a position in which they are hard to maintain. (Limit controls should hold fans off until actuated damper is proved open).


*The Cv coefficient for 50% open dampers is about 5.0. Add fan effect and the minimum pressure to hold the dampers open against gravity to get the (like new) design loss for gravity dampers.