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# Pipelines, Piping and Fluid Mechanics engineering FAQ

## Flow in Pipe

 Friction Factor Expressions - Implicit and Explicit by quark faq378-1236 Posted: 20 Apr 06 (Edited 31 Jul 09) Friction factor is calculated, generally, by any one of the three implicit equations of Colebrook. The equation, I use, is of the form1/(f)1/2 = -2.0log((e/D)/3.7 + 2.51/Re(f)1/2)Note: This is applicable for Re>3000. For laminar flows, Re<2100, Poiseuille's law (f = 64/Re)is usedEngineers, who think that it is difficult to solve this equation, use Moody's chart to get values of 'f'. But, if you go through next FAQ(Spreadsheet for Friction Factors), you will find out how easy it is to solve the implicit equation by some simple calculations in Excel.To make our life easier, some great engineers developed explicit expressions for the friction factor. As I went on reading the subject, I came to know that there were many explicit expressions that equal no. of hairs on my head (no, I am not bald FYI). Out of those, the following are the famous equations.Serghides Equation (for Re>2100 and any e/D)f = [A – [(B-A)2/(C-2B+A)]]-2A = -2.0 log((e/D)/3.7 + 12/Re)B = -2.0 log((e/D)/3.7 + 2.51A/Re)C = -2.0 log((e/D)/3.7 + 2.51B/Re)Moody Equation (40004000 and any e/D)f = 0.094(e/D)0.225 + 0.53(e/D) + 88(e/D)0.44 x Reaa = -1.62(e/D)0.134Jain Equation (for 5000

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