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Fluid friction due to flow comes into play when we try to calculate the pressure drop that is taking place due to the flow. The friction acting is taken into account by friction factor (f). The friction factor is dependent on the Reynolds number of the fluid which is defined as the ratio of the driving force to the viscous flow of the fluid.
The pressure drop is calculated as
The friction factor depends on flow and it has various correlations as far as it has it's calculations. The following information can be found in the "Unit Operations" book by McCabe and Smith. The below information is being taken from wikipedia.