This topic is totally alien to me, so I may be asking something very obvious.

I am designing a cooling system. In the bulk of aluminum, I drill 3mm holes and then I connect them one to another to create a long path for the stream.

The question is now, how long can this maze be? What I am trying to figure out to answer this question is first of all, what pressure is required to drive water into this pipeline. And to determine that, I believe, somewhere should be a coefficient of friction between water and aluminum. Though I can't seem to find a useful chart, it's all about solid bodies friction. Is it because the coefficient might in fact depend on the pipe diameter?

  • $\begingroup$ It is a beam of segments, you need to provide regularly spaced supports to prevent sagging. Slopping the pipe to utilize gravity flow, unless you need a higher flow rate, no pressure is required. Using the friction coefficient for corrugate metals should be giving you a close enough estimate. $\endgroup$
    – r13
    Oct 20, 2021 at 1:49
  • $\begingroup$ It actually should flow regardless orientation. $\endgroup$ Oct 20, 2021 at 3:40
  • 1
    $\begingroup$ Why not a parallel path? $\endgroup$
    – Solar Mike
    Oct 20, 2021 at 4:10
  • 1
    $\begingroup$ This is an ok approach. Might be a pain to maintain long term but sometimes that's not a problem. For rough approximation, use any "pipe flow" calculator on the web, with the total length of the 3mm diameters. Assuming that is the smallest diameter, and the connections between the holes are less restrictive. Also for this purpose, if you want to just build it, it's super easy to measure with a cup.... $\endgroup$
    – Pete W
    Oct 20, 2021 at 8:57
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    $\begingroup$ Yes, I would expect a very large pressure drop (mainly friction loss) due to the size of the pipe. $\endgroup$
    – r13
    Oct 20, 2021 at 13:04

1 Answer 1


The terms you are looking for are Major and Minor pressure losses. Which are a misnomer for systems on the scale your are talking about, since "Minor" losses are usually the greater. The pressure drop across any feature of the system (sudden expansion, sudden contraction, a split, a component like a valve) is:

$\Delta P = 0.5 k\rho Q^2/A^2$

$\rho$ density, $Q$ volumetric flow rate, $A$ cross sectional area. $k$ is a factor you can look up in a table for Minor losses. Valves and filters also have published $C_V$ values, which are used in a different formula to relate pressure drop to flow rate. Major losses are the friction in a straight pipe, and $k=(f*L/D)$ is expressed as a function of friction factor (very conservatively $f=0.1$), length, and diameter. For non-civil engineering flow systems, major losses are often negligible.

Add all these Delta Ps together across the system for the required flow rate, and you can size your pump. Often with a healthy factor, like double.

  • $\begingroup$ Great, thank you for the answer. Is it all in SI units? $\endgroup$ Oct 21, 2021 at 4:08
  • $\begingroup$ @GregoryKornblum use any units you like but just be consistent. $\endgroup$
    – Solar Mike
    Oct 21, 2021 at 5:37
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    $\begingroup$ I forgot a factor of 2 in the formula. Fixed $\endgroup$
    – RC_23
    Oct 21, 2021 at 16:08

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