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If reducing from one pipe size to another through a reducing fitting, say 6.25cm to 5cm, how do I calculate the pressure drop? Also, how does this impact a change in volumetric flow rate (or does it affect the volumetric flow rate)?

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You can calculate the pressure drop with the following formula:

$p_1 - p_2 = \frac {W^2}{2 rho}$*$(\frac {1}{A_2^2} - \frac {1}{A_1^2})$

where $W$ is the mass flow in $[\frac {kg}{s}]$,

$rho$ is the density in $[\frac {kg}{m^3}]$,

$A_1$ and $A_2$ are the cross-sectional areas before and after the reduction.

To answer Your second question: it shouldn't affect the volumetric flow rate, since the cross-sectional area is smaller, the fluid velocity increases.

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  • $\begingroup$ This implies that in an expansion fitting, the pressure rises? It also implies that a reduction followed by an expansion to the same size (or vice versa) would result in no net change. So this is some form of idealized flow, yes? $\endgroup$ – Sherwood Botsford Aug 25 '19 at 14:41

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