Moody's friction factor (also known as the Darcy friction factor) and Fanning friction factor are the two types of friction factor used in this world for pressure drop calculations.
Relationship:
f = Friction factor.
fd = Darcy friction factor
ff = Fanning friction factor.
NOTE: The third friction factor (which is mentioned above in the correlation) is used in Coulson & Richardson Vol. 6 for pressure drop calculation.
In text books you'll come across various pressure drop equations, the only difference lies in the type of friction factor used. Darcy / Moody friction factor is most widely used for pressure drop calculations, as they can be easily interpreted from moody's chart.
To keep it simple and straight forward, note the following points and equations very carefully to avoid any confusion on this topic henceforth.
Equations:
1. Darcy Friction Factor --- Laminar condition
Pressure drop equation:
Moody's Chart representing Darcy Friction factor:
The graph above is a Moody's chart for calculating friction factor using Reynolds Number (Re) and relative roughness. The " f '" on Y-axis is Darcy friction factor and not Fanning friction factor.
TRICK:
Follow the red line marked on the graph, it indicates that for Re = 1000, f = 0.064 which satisfies the Darcy's equation for laminar case. Thus this moody's chart gives us the Darcy friction factor and now I can, without any hesitation, use this " f D " value to calculate the pressure drop by using the corresponding pressure drop equation.
2. Fanning Friction Factor --- Laminar condition
Pressure drop equation:
BEWARE !! Many a times the graph that you might be referring to will give a value of f = 0.016 for Re = 1000 i.e. using f = 16 / Re, so this is fanning friction factor. The following Moody's chart shows the aforementioned case.
3) Friction Factor:
According to Coulson and Richardson vol. 6, we have the following equation for pressure drop calculation and for that particular equation if you are using, make sure you use the correct graph for that equation which is shown below.
NOTE: Its not necessary to have Laminar condition to use this equations / graphs as I have highlighted and focused only on the laminar region. Its just to make my point clear of explaining you the trick to help you to check whether the curve that you are referring to gives you Darcy or Fanning friction value and which graph and equation you need to use for calculation of pressure drop.
I have here made it easy by putting the pressure drop equation inline with their respective graphs.
I Hope, the point of this article is clear and friction factors won't be any confusion in any minds from now on. I follow the same practice while determining the value of friction factor and this check has never failed me.
Fix your steps of calculating pressure drop, but I insist on using Darcy equation i.e. Darcy friction factor. This should not lead to a conclusion that fanning friction factor should not be used. Remember either of it can be used, and both will lead to correct pressure drop, provided you use the right set of equations and keep the relationship in mind
Just in case you get confused again, you can re-visit this article and if you have any doubts in mind you can leave a comment here.
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