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it isn't constant ....every building is different ....you the cfm form AHU and the air velocity to get the friction
Typical range of design equal friction loss rates used for Low Velocity systems are from0.08 to0.15" w./100 ft of duct.
For High Velocity systems, friction loss rate may be as high as0.6" w./100 ft.
Now the exact friction value is in this range is the compromise between optimized friction loss and economical duct sizing. So a nominal value can be used which varies case to case of projects.
You can use any value below0.1" WG for low velocity duct sizing calculation. for example0.075" wg/100 FT.
