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The Darcy-Weisbach Equation used in conjuction with the Moody Diagram (for obtaining Fanning friction factors) is considered by many people to be the most reliable method for calculating either frictional pressure loss or frictional head loss for the turbulant flow of incompressible fluids in pipes.
The Darcy-Weisbach Equation for pressure loss due to friction (in the SI metric system) is:
[Δ]P = 4 f ( L / D ) ( [ρ] v[sup]2[/sup] ) / 2
where:
[Δ]P = pressure loss, in Pascals
f = Fanning friction factor (dimensionless) from Moody's diagram
L = pipe length, in meters
D = pipe diameter, in meters
[ρ] = fluid density in kilograms per cubic meter
v = fluid velocity, in meters per second
And for head loss due to friction, the equation is:
[Δ]h = 4 f ( L / D ) ( v[sup]2[/sup] ) / ( 2 g )
where:
[Δ]h = head loss, in meters
f = friction factor (dimensionless)
L = pipe length, in meters
D = pipe diameter, in meters
v = fluid velocity, in meters per second
g = local gravitational acceleration of 9.807 m/sec[sup]2[/sup]
Make sure that you are using the Fanning friction factors and not the Darcy friction factors (which are 4 times as large as the Fanning factors).
Milton Beychok
(Visit me at www.air-dispersion.com)
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