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Chemical plant design & operations FAQ

Hydraulic Analysis -fluid mech

What is the difference between Fanning and Moody friction factors? by chemebabak
Posted: 6 Oct 11


Many folks calculate 4 times greater head loss (or 4 times less) than the actual friction loss.  This comes from confusion between Moody and Fanning Friction factors.  Some friction factor graphs are for Moody Friction factor, which is 4 times Fanning friction factor.  That is, f = 64/Re is Moody and f = 16/Re is Fanning.

Be careful with your hydraulic calcs.  It is easy to mix the two and calculate 400% greater (or 25% less) head loss.  The calculation for head loss in feet is:

using Moody Friction factor -
h(friction) = f(M) * (L/D) * v^2 / (2 * g)

using Fanning Friction factor -
h(friction) = 4*f(F) * (L/D) * v^2 / (2 * g)

where,
h(friction) = head loss by friction in feet
f(M) = Moody Friction factor
f(F) = Fanning Friction factor
L = length in feet
D = pipe inside diameter in feet
v = velocity in ft/s
g = 32.174 ft/s^2, acceleration due to gravity

The Colebrook-White equation is an iterative method that calculates Fanning friction factor.
f(F)^2 = 1 / ( -4 * Log(eps / (3.7 * D) + 1.256 / (Re * √f(F) )

where,
eps = pipe roughness in feet
Re = Reynold's number


 

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