Coeficientes de perda menor (pressão ou perda de carga) para componentes do duto de ar.
Minor or dynamic losses in duct systems are pressure losses caused by
- mudança na direção do ar de cotovelos, deslocamentos e decolagens
- restrições ou obstruções no fluxo de ar – ventiladores de entrada/saída, amortecedores, filtros e bobinas
- changes in air velocity due to changes in duct sizes
The minor or dynamic pressure drop in the air duct system components can be expressed as
Δ p minor_loss = ξ ρ v 2 / 2 (1)
Where
X = coeficiente de perda menor
Δ p minor_loss = menor perda de pressão (Pa (N/m 2 ), psf (lb/ft 2 ) )
r = densidade do ar (1,2 kg/m 3 , 2,336 10 -3 slugs /ft 3 )
v = flow velocity (m/s, ft/s)
- perda menor também pode ser expressa como head water column – como mm H2O ou polegadas H2O
Coeficientes de perdas menores para diferentes componentes em sistemas de distribuição de dutos de ar:
| Component or Fitting | Minor Loss Coefficient – ξ – |
|---|---|
| 90 O curva, afiada | 1.3 |
| 90 O curva, com palhetas | 0,7 |
| 90 O curve, rounded radius/diameter duct <1 | 0,5 |
| 90 O curved, rounded duct radius/diameter >1 |
0,25 |
| 45 O curve, sharp | 0,5 |
| 45 O curve, rounded radius/diameter duct <1 |
0,2 |
| 45 O curved, rounded duct radius/diameter >1 |
0,05 |
| T, flow to branch (applied to branch speed) |
0,3 |
| Duct flow to the room | 1,0 |
| Flow from room to duct | 0,35 |
| reduction, tapered | 0 |
| widening, abrupt (due to speed before reduction) (v 1 = velocity before widening and v 2 = velocity after flare) |
(1 – v 2 / v 1 ) 2 |
| Enlargement, tapered angle < 8 O (due to speed before reduction) (v 1 = speed before magnification and v 2 = speed after magnification) |
0,15 (1 – v 2 / v 1 ) 2 |
| Magnification, tapered angle > 8 O (due to speed before reduction) (v 1 = speed before magnification and v 2 = speed after magnification) |
(1 – v 2 / v 1 ) 2 |
| Grilles, ratio of free area 0.7 to total surface | 3 |
| Grilles, 0.6 free area ratio to total surface | 4 |
| Grilles, 0.5 free area ratio to total surface | 6 |
| Grilles, 0.4 ratio of free area to total surface | 10 |
| Grilles, 0.3 ratio of free area to total surface | 20 |
| Grilles, 0.2 free area ratio to total surface | 50 |
Example – Minor Loss on a Curve
The smallest loss on a closed curve of 90 O with a loss coefficient of less than 1.3 and air speed 10 m/s can be calculated as
Δ p minor_loss = (1,3) (1,2 kg/m3 ) (10 m/s ) 2/2
= 78 (N/m2 , Pa)
Source: https://www.engineeringtoolbox.com/minor-loss-air-ducts-fittings-d_208.html
