Lower loss coefficients (pressure or load loss) for air duct components.
Smaller or dynamic losses in duct systems are pressure losses caused by
- change in the air direction of elbows, displacements and takeoffs
- Restrictions or obstructions on airflow - input/output fans, shock absorbers, filters and coils
- changes in air speed due to changes in duct sizes
The loss of pressure lower or dynamic in the components of the air duct system can be expressed as
D p Loss_Menor = x r v 2 / 2 (1)
Where
h = minor loss coefficient
D p minor_perda = lower pressure loss (PA (n/m 2 ), psf (lb/ft 2 ) )
r = Air density (1.2 kg/m 3 , 2,336 10 -3 slugs /ft 3 )
v = Flow speed (m/s, feet/s)
- minor loss can also be expressed as headwater column - Like mm H2O or inch H2O
Coefficients of smaller losses for different components in air duct distribution systems:
| Component or fitting | Minor loss coefficient - X - |
|---|---|
| 90 o curve, sharp | 1.3 |
| 90 o curve, with reeds | 0,7 |
| 90 o curva, duto de raio/diâmetro arredondado <1 | 0,5 |
| 90 o curve, rounded duct of raio/diâmetro >1 |
0,25 |
| 45 o bend, sharp | 0,5 |
| 45 o curva, duto de raio/diâmetro arredondado <1 |
0,2 |
| 45 o curve, rounded duct of raio/diâmetro >1 |
0,05 |
| T, flow to the extension (applied to velocity in the extension) |
0,3 |
| Duct flow to the room | 1,0 |
| Room flow to the duct | 0,35 |
| Reduction, tapered | 0 |
| Enlargement, abrupt (due to speed before reduction) (v 1 = Speed before enlargement and v 2 = speed after enlargement) |
(1 – v 2 / v 1 ) 2 |
| Alargamento, ângulo cônico < 8 o (due to speed before reduction) (v 1 = Speed before expansion and v 2 = speed after expansion) |
0,15 (1 – v 2 / v 1 ) 2 |
| Ampliação, ângulo cônico > 8 o (due to speed before reduction) (v 1 = Speed before expansion and v 2 = speed after expansion) |
(1 – v 2 / v 1 ) 2 |
| Grids, proportion of free area of 0.7 for total surface | 3 |
| Grids, proportion of free area of 0.6 for total surface | 4 |
| Grids, proportion of free area from 0.5 for total surface | 6 |
| Grills, 0.4 proportion of free area compared to the total surface | 10 |
| Grills, 0.3 proportion of free area compared to the total surface | 20 |
| Grids, proportion of free area from 0.2 for total surface | 50 |
Example - smaller loss in a curve
The smaller loss in a closed curve of 90 o with lower loss coefficient 1.3 and air speed 10 m/s can be calculated as
D p Loss_Menor = (1,3) (1,2 kg/m3 ) (10 m/s ) 2/2
= 78 (N/m2 , Pa)
Source: https://www.engineeringtoolbox.com/minor-s-ir-ductts-fittings-d_208.html
