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# Air Duct Components - Minor Dynamic Loss Coefficients

## Minor loss (pressure or head loss) coefficients for air duct components.

Minor or Dynamic losses in duct systems are pressure losses caused by

• change in air direction from elbows, offsets, and takeoffs
• restrictions or obstructions in the air stream - in/outlet fans, dampers, filters, and coils
• air velocity changes due to changes in duct sizes

Minor or dynamic pressure loss in air duct system components can be expressed as

Δpminor_loss = ξ ρ v2 / 2                            (1)

where

Δpminor_loss = minor pressure loss (Pa (N/m2), psf (lb/ft2))

ρ = density of air (1.2 kg/m3, 2.336 10-3 slugs/ft3)

v = flow velocity (m/s, ft/s)

Minor loss coefficients for different components in air duct distribution systems:

Component or FittingMinor Loss Coefficient
- ξ -
90o bend, sharp 1.3
90o bend, with vanes 0.7
90o bend, rounded
0.5
90o bend, rounded
0.25
45o bend, sharp 0.5
45o bend, rounded
0.2
45o bend, rounded
0.05
T, flow to branch
(applied to velocity in branch)
0.3
Flow from duct to room 1.0
Flow from room to duct 0.35
Reduction, tapered 0
Enlargement, abrupt
(due to speed before reduction)
(v1= velocity before enlargement and v2 = velocity after enlargement)
(1 - v2 / v1)2
Enlargement, tapered angle < 8o
(due to speed before reduction)
(v1= velocity before enlargement and v2 = velocity after enlargement)
0.15 (1 - v2 / v1)2
Enlargement, tapered angle > 8o
(due to speed before reduction)
(v1= velocity before enlargement and v2 = velocity after enlargement)
(1 - v2 / v1)2
Grilles, 0.7 ratio free area to total surface 3
Grilles, 0.6 ratio free area to total surface 4
Grilles, 0.5 ratio free area to total surface 6
Grilles, 0.4 ratio free area to total surface 10
Grilles, 0.3 ratio free area to total surface 20
Grilles, 0.2 ratio free area to total surface 50

### Example - Minor Loss in a Bend

The minor loss in a 90o sharp bend with minor loss coefficient 1.3 and air velocity 10 m/s can be calculated as

Δpminor_loss = (1.3) (1.2 kg/m3) (10 m/s)2 / 2

= 78 (N/m2, Pa)

## Related Topics

• Fluid Mechanics - The study of fluids - liquids and gases. Involving velocity, pressure, density and temperature as functions of space and time.
• Ventilation - Systems for ventilation and air handling - air change rates, ducts and pressure drops, charts and diagrams and more.

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Temperature

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Length

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Area

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Volume

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Weight

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Velocity

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Pressure

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mm H2O
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Flow

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cfm

12 1