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# 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

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:

Air Duct Components - Minor Dynamic Loss Coefficients
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.

## Related Documents

### Air Ducts - Minor Loss Coefficient Diagrams

Minor loss coefficient diagrams for air ductwork, bends, expansions, inlets and outletsĀ  - SI units.

### Air Ducts - Sizing

Required duct area vs. air flow.

### Air Ducts - Velocity Diagram

Air flow volume, duct size, velocity and dynamic pressure.

### Duct Sizing - the Equal Friction Method

The equal friction method for sizing air ducts is easy and straightforward to use.

### Ductwork - Seal Classes

Ductwork air leakage classes.

### Equivalent Length vs. Minor Pressure Head Loss in Pipe and Duct Components

Minor pressure and head loss in pipes vs. equivalent length in tubes and duct systems.

### Pipe and Tube System Components - Minor (Dynamic) Loss Coefficients

Minor loss coefficients for components used in pipe and tube systems.

### Ventilation Components - Minor Pressure Loss

Pressure loss in ventilation system components like dampers, filters, heaters, coolers and more.

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