Friction Loss in Ducts
Friction loss or major loss in ducts - equations and online calculator for rectangular and circular ducts - imperial units
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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)
where
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.
Air Ducts Friction Loss Calculator
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) | ||||||
| 4 | 5 | 6 | 8 | 10 | 12 | 16 | |
| 100 | 0.65 | 0.21 | 0.09 | 0.02 | 0.01 | ||
| 1146 | 733 | 509 | 286 | 183 | |||
| 200 | 0.8 | 0.32 | 0.08 | 0.02 | 0.01 | ||
| 1467 | 1019 | 573 | 367 | 255 | |||
| 400 | 1.19 | 0.28 | 0.09 | 0.04 | 0.01 | ||
| 2037 | 1146 | 733 | 509 | 286 | |||
| 800 | 0.34 | 0.14 | 0.03 | ||||
| 1467 | 1019 | 573 | |||||
| 1600 | 0.12 | ||||||
| 1146 | |||||||
The air velocity should not exceed certain limits to avoid unacceptable noise generation.
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