Approximation of friction factor for Laminar Flow

[Dong Aleta]

Hi!

I read from Perry Green's ChE Handbook that the friction factor for Re ≤ 2,100 can be approximated by ƒ = 16/Re. But there was this question that I encountered (though I don't know the source) and according to it, ƒ = 64/Re for laminar flow. Can someone clarify which is which? Thank you!

 

[stockzahn]

The formula f=64/Re is called the Darcy friction factor and is derived from f=(Δp⋅2⋅D)/(L⋅ρ⋅u²)

The formula f=16/Re is called the Fanning friction factor and is derived from f=(Δp⋅R)/(L⋅ρ⋅u²)

Basically they are the same, but different values/dimensions are used to define them

 

[Dong Aleta]

The formula f=64/Re is called the Darcy friction factor and is derived from f=(Δp⋅2⋅D)/(L⋅ρ⋅u²)

The formula f=16/Re is called the Fanning friction factor and is derived from f=(Δp⋅R)/(L⋅ρ⋅u²)

Basically they are the same, but different values/dimensions are used to define them

I see. That's really confusing. Anyway, thank you!