Air Duct Components - Minor Dynamic Loss Coefficients
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 Fitting | Minor 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 |
Download and print Air Flow - Minor Loss chart
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
Design of systems for ventilation and air handling - air change rates, ducts and pressure drops, charts and diagrams and more.
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