Straight duct
Loss of linear pressure
The loss of linear pressure, due to the friction of a fluid flowing through a duct, can be calculated using the
Darcy equation.
Where:
∆Pf/L
Loss of linear pressure = 2.3 Pa/m
f
Friction factor = 0.0191
Pv
Velocity pressure = 31 Pa
Dh
Hydraulic diameter = 254.5 mm
2.3
0.95 Pa/m
Ref: 'ASHRAE Duct Design'
Friction factor
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For turbulent flow, the friction factor can be obtained using the Altshul-Tsal formula.
Where:
f
Friction factor = 0.0191
f
Provisional friction factor = 0.0191
ε
Roughness coefficient = 0.09 mm
Dh
Hydraulic diameter = 254.5 mm
Re
Reynolds's number = 120727
Reynolds's number
For standard air and temperature between 4 and 38¡ãC (40 and 100¡ãF), Reynolds number can be calculated
by:
Where:
Re
Reynolds's number = 120727
Dh
Hydraulic diameter = 254.5 mm
v
Velocity = 7.14 m/s
Ref: 'ASHRAE Duct Design'
Hydraulic diameter
From experiments using round, square, and rectangular ducts having essentially the same hydraulic
diameter, Huebscher (1948) found that each, for most purposes, had the same flow resistance at equal
mean velocities.
Where:
Dh
Hydraulic diameter = 254.5 mm
A
Section = 0.07 m
P
Perimeter = 1100 mm
Ref: 'ASHRAE Duct Design'
Diameter equivalent round
Huebscher (1948) developed the relationship which establishes the size equivalencies between rectangular
and round ducts based on equal flow, resistance, and length.
Where:
De
Diameter equivalent round = 286.4 mm
W
Width = 350 mm
H
Height = 200 mm
Ref: 'ASHRAE Duct Design'
Velocity
The velocity is calculated using the following expression:
Produced by a non professional version of CYPE
Where:
v
Velocity = 7.14 m/s
Q
Flow = 500 l/s
A
Section = 0.07 m
7.14
10 m/s
Ref: 'ASHRAE Duct Design'
Velocity pressure
When the fluid flows along the duct, its movement produces an additional increment of pressure due to it
colliding against a perpendicular surface.
Where:
Pv
Velocity pressure = 31 Pa
ρ
Air density = 1.2 kg/m
v
Velocity = 7.14 m/s
Ref: 'ASHRAE Duct Design'
Section
Rectangular ducts
Where:
A
Section = 0.07 m
W
Width = 350 mm
H
Height = 200 mm