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This is a question that has been haunting me my entire career. The pressure loss caused by friction is not the same as the pressure "loss" caused by vacuum, right?

I mean, if I have a gas flowing though a pipe, the more restricted the way is, the bigger is the pressure drop caused by the pipe and therefore it should be more difficult for me to push the gas through said pipe.

Maybe the best way to put it is: what is the difference between a 2000 Pa pressure drop caused by friction compared to using a pump to pull a gas with a suction pressure of -2000 Pa? Why does one of them block my gas flow and the other helps it flowing?

Thanks in advance, this question drives me crazy

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  • $\begingroup$ which one blocks the flow? $\endgroup$
    – jsotola
    Commented Feb 21 at 17:44
  • $\begingroup$ Flow-vs-pressure_drop is exact same in concept. Only thing to consider is that the density and viscosity might be different in the case of (+P to 0) vs (0 to -P) ... so that would give a slightly different flow in the two cases, obviously more significant for larger P. $\endgroup$
    – Pete W
    Commented Feb 21 at 18:13
  • $\begingroup$ @jsotola The pressure drop caused by obstacles in the pipe is the one that blocks the flow (for example a packed material). $\endgroup$
    – lexax127
    Commented Feb 21 at 19:14
  • $\begingroup$ which one is it? ... the one with the compressor, or the one with the vacuum pump? $\endgroup$
    – jsotola
    Commented Feb 21 at 19:54

2 Answers 2

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Consider the fluid as a control volume -- a defined size of the fluid. Pipe friction works at the surface of the control volume to prevent the entire fluid as a unit from moving through the pipe in its desired direction. Pumping (suction as you phrase it) works at the surface of the control volume to move the fluid in a specific direction.

Alternatively, recall from physics that a friction vector goes in the opposing direction to the motion of the object. You push (or pull) a block along a surface, and friction pushes in the opposing direction. Both forces do work on the object. But one does work in the direction of the force (pumps), while the other does work in the opposing direction (friction).

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When you compare two steady state situations with a horizontal pipe, where in the first, a pump is pumping a fluid into the pipe from left and in the other, there is suction pump at the right end which is sucking the same fluid, do you think the fluid in the middle will know the difference? In both cases, the fluid will flow from left to right, pushed by the pressure difference, overcoming friction loses.

Another way to look at it is that if you want to get a fluid moving in a pipe, you have to create pressure difference. And to move it in one direction, that means either lowering the pressure at one end or increasing it at the other (or combination of both).

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  • $\begingroup$ Thanks for your answer! What I find troubling me is the fact that: The more suction you apply, the more fluid will flow. But the more pressure drop (caused by obstacles in the pipe, friction or packing material) does not mean that the fluid will flow more, but the exact contrary. That is my struggle $\endgroup$
    – lexax127
    Commented Feb 21 at 19:12
  • $\begingroup$ There is a difference between "pressure loss" and "pressure difference". In order to get the fluid moving through a pipe with constant cross section, you need to create pressure difference. When the fluid is moving through the pipe, some of the internal energy is lost due to friction, which leads to pressure drop. When there is a pressure difference between the ends of a horizontal pipe and the fluid is not moving initially, it will start accelerating and eventually, a steady state is reached where the pressure difference. between the ends will be the same as the pressure loss. $\endgroup$
    – Tomáš Létal
    Commented Feb 22 at 16:42

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