System effect factor. How it affects operating cost.

The SEF (system effect factor) causes loss of capacity of fan volume attributed to poorly designed and installed duct fittings at the inlet or outlet fan. It is the interaction of the air with obstructions such as elbows, guards, drive sheaves or dampers, that create SEF.
So SEF must be considered at the design stage of a fan system, it must be included as a separate item in the fan equipment schedule. Ignoring SEF has heavy cost implications.