A fan does not deliver its maximum catalog airflow at every pressure. Its actual operating point is where the fan curve meets the resistance curve of the installed system.
Where resistance comes from
Straight ducts, elbows, transitions, branches, grilles, filters, dampers, coils, silencers, collectors, and discharge terminals all add pressure loss. Dirty filters and process buildup may increase resistance further over time.
Why open-air ratings can mislead
A fan selected only from its free-air volume may deliver much less airflow after it is connected to a real duct system. The pressure requirement must therefore be calculated or estimated before confirming the model.
Useful information for review
- Duct dimensions, material, and total length
- Quantity and type of fittings
- Filters, dampers, silencers, coils, and collectors
- Required terminal or capture velocity
- Expected clean and dirty system conditions