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Air Duct Components and Minor Dynamic Loss Coefficients

Minor loss - pressure or head loss - coefficients for air duct distribution systems components

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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

ξ = minor loss coefficient

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
radius/diameter duct <1
0.5
90o bend, rounded
radius/diameter duct >1
0.25
45o bend, sharp 0.5
45o bend, rounded
radius/diameter duct <1
0.2
45o bend, rounded
radius/diameter duct >1
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)

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