if I have a closed system of horizontal pipes, filled with water, where I want to create a flow at some rate by using a pump. Does the length of the system influences the flow ? (in real world not in the wonderful world of the books)
Answer
Yes, the flow rate is related to the pump pressure by the Darcy-Weisbach equation:
$$ \Delta P = f_d \frac{L}{D} \frac{\rho v^2}{2} $$
where $L$ is the length of the pipe, $D$ is the diameter, $\rho$ is the water density, $v$ is the flow velocity and $f_D$ is a fudge factor called the Darcy friction factor. $f_D$ varies with the pipe diameter, density and flow rate, but we have empirical formulae for calculating $f_D$ over a wide range of conditions.
So provided you keep everything else constant the pressure drop is proportional to the pipe length.
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