What Is the Fanning Friction Factor?
The Fanning friction factor is an element in the calculation of pressure loss due to friction in a pipe. It is a function of the roughness of the pipe and the level of turbulence within the liquid flow. These factors can be determined experimentally but are more often taken from charts and diagrams. The numbers are dimensionless, meaning they have no units of measure.
The pressure of liquid flowing through a pipe decreases due to friction between the interior walls of the pipe and the moving liquid. Either pumps or gravity must provide the energy to move liquid. In very long pipes, the pressure drop due to frictional loss will be so high that the liquid will not flow at all. Pipelines, such as the Alaska oil pipeline, require intermediate pumping stations to boost pressure.
An understanding of the pressure loss that occurs as liquids move through pipes is essential in any piping application. It is critical to chemical processes that use pipes as tubular flow reactors. Pipes used as reactors produce reaction conditions in which temperature and pressure are easily controlled. Reaction residence time and degree of reaction completion is a function of the length of the pipe.
Exothermic reactions give off heat as they progress. To maintain isothermal conditions, and a constant Fanning friction factor, the pipe will have to be cooled in a counter-current direction. Endothermic reactions, which absorb heat, will require the opposite treatment. If isothermal conditions are not maintained, the calculations using the Fanning friction factor will have to satisfy the change in viscosity and friction that occurs as the liquid gets warmer or cooler.
Reynolds numbers are dimensionless measures of the degree of turbulence in the liquid. In laminar flow with Reynolds numbers less than 2,000, the liquid moves with a bullet-shaped velocity profile and little intermixing. The maximum velocity occurs in the center of the pipe cross-section and is twice the average flow of the liquid. Turbulent flow, with complete mixing, occurs at Reynolds numbers above 3,000. A thin buffer zone, with Reynolds numbers between 2,000 and 3,000, occurs between the laminar and turbulent zones.
A Fanning friction factor can be determined by measuring pressure drops across piping that is large enough in diameter to be scalable for field or plant operations. Typically, these experiments are performed if laminar flow conditions are needed. More commonly, the Fanning friction factor is read from a chart, as most plug flow reactors are operated at high Reynolds numbers.
The roughness of the surface of the pipe interior is determined by measurement. The Reynolds number is calculated from the pipe diameter, fluid viscosity, and pressure drop. Plots of the Fanning friction factor against the Reynolds number for pipes of varying roughness are available in engineering handbooks. These books also have tables of the surface roughness of various materials.
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