Select the appropriate DC fan to supply 400 LFM (2 m/s) airflow into a 4 inch (102 mm) diameter circular duct with a pressure drop of 0.1 inches of water (25 Pa).
Using the airflow calculator above, type the airflow value of 400 and select LFM as the units. Next type the circular duct dimensions of r = 2 in. After pressing the Calculate Airflow button, read the CFM requirement of 34.91 CFM in the volume airflow units column.
This value coupled with the pressure drop of 0.1 inches of water (25 Pa) now allows the use of standard fan curves. For example, click the following link to select a 12 volt DC fan, which meets the requirement: 12 DC Fan. Notice that the free air delivery of fans is the airflow delivered with no resistance. The fourth fan listed, Flight II 120 - FT12B3, would deliver 37.8 CFM into the duct and 90 CFM if no resistance was presented.