The major loss, or friction loss, in a circular duct in galvanized steel with turbulent flow can for imperial units be expressed
Δp = (0.109136 q1.9) / de5.02 (1)
Δp = friction (head or pressure loss) (inches water gauge/100 ft of duct)
de = equivalent duct diameter (inches)
q = air volume flow - (cfm - cubic feet per minute)
For rectangular ducts the equivalent diameter must be calculated.
The friction loss calculator below is based on formula (1):
Pressure loss and air flow velocity for some common duct sizes and air flow volumes can be taken from the table below:
|Pressure Loss (inches water gauge per 100 feet duct)|
|Air velocity (ft/min)|
|Air Volume (cfm)||Duct Size (inches)|
The air velocity should not exceed certain limits to avoid unacceptable noise generation.