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The Fanning friction factor is used to calculate pressure loss in pipes due to friction. It is determined by the roughness of the pipe and turbulence of the liquid flow. Reynolds numbers measure turbulence. Understanding pressure drop is crucial in piping applications, especially in chemical processes. Isothermal conditions must be maintained for accurate calculations. The Fanning friction factor can be determined experimentally or from graphs and charts.
The Fanning friction factor is an element in calculating the 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 graphs and charts. Numbers are dimensionless, meaning they have no units of measurement.
The pressure of liquid flowing through a pipe decreases due to friction between the inner walls of the pipe and the moving liquid. Both pumps and gravity must provide the energy to move the liquid. In very long pipes, the pressure drop due to friction loss will be so great that the liquid will not flow at all. Pipelines, such as the Alaska Pipeline, require intermediate pumping stations to build up pressure.
Understanding the pressure drop that occurs as liquids move through pipes is essential in any piping application. It is critical for chemical processes that use tubes such as tubular flow reactors. Pipes used as reactors produce reaction conditions where temperature and pressure are easily controlled. Reaction residence time and degree of reaction completion are a function of tube length.
Exothermic reactions give off heat as they progress. To maintain isothermal conditions and a constant Fanning friction factor, the pipe will need to be cooled in the counterflow direction. Endothermic reactions, which absorb heat, will require the opposite treatment. If isothermal conditions are not maintained, calculations using the Fanning Friction Factor will have to accommodate for the change in viscosity and friction that occurs as the liquid heats up or cools.
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 mixing. The maximum velocity occurs in the center of the pipe section and is twice the average liquid flow rate. 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 fan friction factor can be determined by measuring the pressure drops across piping of a large enough diameter to be scalable for field or plant operations. Typically, these experiments are performed if laminar flow conditions are required. Most commonly, the Fanning friction factor is read from a graph, since most plug flow reactors operate at high Reynolds numbers.
The roughness of the inner surface of the pipe is determined by measurement. The Reynolds number is calculated from the diameter of the pipe, the viscosity of the fluid and the pressure drop. Graphs of friction factor Fanning versus Reynolds number for pipes of various roughnesses are available in engineering manuals. These books also have tables of the surface roughness of various materials.
