Darcy Equation

Helloy
I found in "Chemical Engineering" book (R.K Sinnott, volume 6, third edition, page 200) a Darcy equation in unusual interpretation (at least for me)
Pressure drop=8*f (L/Di)*r*v^2/2
Where f= friction factor
I'd like to know ,What is factor "8" ?
and another question, What is difference between a Fanning friction factor and Darcy friction factor?
Thanks in advance
bmicky

The form I usually use is:

Delta(P) = 4*(phi)*(l/d)*(rho)*u^2

Where phi is a friction factor, = f/2.

The factor 8 (or 4 in the equation above) is just a coefficient. If you use the Moody friction factor (f'), you leave out the 4 in the equation above, since the Moody friction factor is 4 times greater than the Darcy and Fanning friction factor.

The various friction factors are given by the following equations:

Moody = 8R/{(rho)*u^2}
Fanning = 2R/{(rho)*u^2}
Darcy = 2R/{(rho)*u^2}

where (rho) is the density, u is the mean velocity (or superficial velocity if you're doing two phase flow calculations), R is the shear stress.

As you can see, the Darcy and Fanning friction factors are calculated in the same way, so there isn't any practical difference.

You just need to be careful when using pipe friction charts - I've got pipe friction charts giving (phi) and charts giving f as the friction factors, and they are different by a factor of 2.

Dave