B&C Technologies DI-475 Industrial - Manual (Page 14)

Outside Duct Work Protection
To protect the outside end of horizontal duct work from the weather, a 90 o
elbow bent downward should be installed where the exhaust exits the building.
If the duct work travels vertically up through the roof, it should be protected from
the weather by using a 180 o turn to point the opening downward. In either case,
allow at least twice the diameter of the duct between the duct opening and the
nearest obstruction.
IMPORTANT: Do not use screens or caps on the outside of opening
of exhaust duct work.