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Eq. (19-1) has been determined by dimensional analysis (see Reference ). The friction factor is a function of Reynolds number, ReD = pVmD∕μ and pipe inner surface roughness, e∕D. The friction factors, f, for fully developed pipe flows have been obtained experimentally for different Reynolds numbers and pipe inner surface roughness conditions, and they are given in References  and  as graphs, called Moody Diagrams, or as empirical equations.