Engineering
& Technical
Data
R-14
Darcy
Wiesbach
Equation
Darcy-Weisbach Equation
Where ƒ is an empirically-derived coefficient which is
a function of Reynold’s Number and relative roughness
of pipe. ƒ can be determined with the use of the Moody
Diagram (shown below).
Henry Darcy and Julius Weisbach
h f = ƒ × L × V2
D
2g
• Re: (Reynold’s Number) = VD
V
• Valid for laminar or turbulent flow regimes.
• Valid only for circular pipes flowing full.
Where: hf = Head loss created by viscous effects (ft.)
ƒ = D-W non-dimensional resistance
coefficient
L = Length of conduit (ft.)
D = Diameter of conduit (ft.)
V2 = Velocity head (ft.)
2g
• K s (Relative Roughness) = Ks
D
D
Moody Diagram
Re ƒ½ = D
v
3/2
103
2
4
6 810 4
2
4
(2ghƒ
)
L
½
6 8 10 5
2
4
6
8106
2
4
6 8
Resistance coefficient, ƒ
Relative roughness, ks
D
Complete turbulence, rough pipes
Boundary Material
Glass, plastic
Copper or brass tubing
Wrought iron, steel
Asphalted cast iron
Galvanized iron
Cast iron
Concrete
103
2
In Millimeters
Smooth
1.5 x 10-3
4.6 x 10-2
0.12
0.15
0.26
0.3 to 3.0
4
6
8 104
Equivalent Sand
roughness, ks
(In Feet)
(Smooth)
(5 x 10 -6)
(1.5 x 10-4)
(4 x 10-4)
(5 x 10-4)
(8.5 x 10-4)
(10-3 to 10-2)
2
Smooth Pipe
4
6
8 105
2
R-14
4
6
8 106
2
4
6
8 107
Re = VD
v
Phone: 800-EJP-24HR (357-2447) • Fax: 207-582-5637
2
4
6
8 108