If I am carrying out a reaction by flowing 200 SCCM of a gas in a 20mm diameter pipe at 100mm length, if I change the pipe diameter to 10mm:

  1. Do I adjust the mass flow rate of the gases?
  2. If so, how do you calculate this?

Thank you for helping. Looking to learn. Cheers


1 Answer 1


It depends on if you have a smooth laminar flow with little friction and steady state flow, or transient, and or turbulent flow.

Assuming you pipe lining is smooth, by changing the inside diameter of the pipe to 10mm, you get the same mass flow rate but at four times the speed. Bernoulli equation applies, conservation of kineticand potential energy.

But if your pipe lining is rough, then you may need to use Poiseuille's Law if the flow is laminar:

$ Flow = \frac {P_1 -P_2}{R} \quad, \ R = \frac{8\eta*L}{\pi*r^4} \quad \eta = viscosity $

In many practical cases both rules apply, meaning you have pressure loss due to viscosity and you have pressure loss to constrictions, however for a short length as 100mm the friction loss is just nominal and the flow rate is constant.

  • $\begingroup$ As this is my first post, I can not upvote your response. Thank you for your time and help. $\endgroup$
    – Luck
    Feb 1, 2019 at 4:34
  • 1
    $\begingroup$ @DanE. You should be anle to accept the answer with the check mark... $\endgroup$
    – Solar Mike
    Feb 1, 2019 at 5:02

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