Friction loss
[1] Friction loss is a significant engineering concern wherever fluids are made to flow, whether entirely enclosed in a pipe or duct, or with a surface open to the air.
In this expression, the properties of the fluid itself are reduced to the kinematic viscosity ν where The friction loss in uniform, straight sections of pipe, known as "major loss", is caused by the effects of viscosity, the movement of fluid molecules against each other or against the (possibly rough) wall of the pipe.
From experimental measurements, the general features of the variation of fD are, for fixed relative roughness ε / D and for Reynolds number Re = V D / ν > ~2000,[a] The experimentally measured values of fD are fit to reasonable accuracy by the (recursive) Colebrook–White equation,[12] depicted graphically in the Moody chart which plots friction factor fD versus Reynolds number Re for selected values of relative roughness ε / D. In a design problem, one may select pipe for a particular hydraulic slope S based on the candidate pipe's diameter D and its roughness ε.
The following table gives Reynolds number Re, Darcy friction factor fD, flow rate Q, and velocity V such that hydraulic slope S = hf / L = 0.01, for a variety of nominal pipe (NPS) sizes.
Also note that the given fD in this table is actually a quantity adopted by the NFPA and the industry, known as C, which has the customary units psi/(100 gpm2ft) and can be calculated using the following relation: where
is the length of the pipe in 100ft Friction loss takes place as a gas, say air, flows through duct work.
First, select the desired pressure loss Δp / L, say 1 kg / m2 / s2 (0.12 in H2O per 100 ft) on the vertical axis (ordinate).
will result in a loss Δp / L of 0.02 kg / m2 / s2 (0.02 in H2O per 100 ft), illustrating the great gains in blower efficiency to be achieved by using modestly larger ducts.
The following table gives flow rate Q such that friction loss per unit length Δp / L (SI kg / m2 / s2) is 0.082, 0.245, and 0.816, respectively, for a variety of nominal duct sizes.
Note that, in approximation, for a given value of flow volume, a step up in duct size (say from 100mm to 120mm) will reduce the friction loss by a factor of 3.
