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Resistance in pressure vs. vacuum

  1. Jan 8, 2005 #1
    Hi :shy:

    I'm working on a project and may be in over my head just a bit :grumpy: I've reached a dilemma, and could really use some help. Here it is:

    An air pump has ducting to its entrance, and when the pump is operational this ducting is experiencing a fixed vacuum. Conversely, there is exit ducting from the air pump experiencing a fixed pressure at the same time. If there is a restriction placed in the entrance ducting (vacuum), then there will be a volume (and pressure/efficiency?) loss at the pump. The main question is: If the same restriction is moved from the vacuum (entrance) side of the pump, to the pressure side (exit), will the volume (and pressure/efficiency?) loss be the same?

    Ant tech would be greatly appreciated, and thank you in advance :smile:
  2. jcsd
  3. Jan 9, 2005 #2


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    What kind of pump is it? Positive displacement or centrifugal?
  4. Jan 9, 2005 #3
    Actually, that may be part of the problem :grumpy: Would the type of pump matter in theory, and if so, why? There are two design models - one with a positive displacement pump, having the restriction only on the intake side; and another with a centrifugal pump, having the (identical) restriction only on the pressure - or outlet side. Each pump produces the same peak pressure.

    I didn't think it would matter how the pressure was made (pump type?); if the fixed restriction is the same, either fore or aft, wouldn't the volume reduction also be fixed?

    Thanks for taking an interest :smile:
  5. Jan 11, 2005 #4


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    Sorry it took so long to get back to you.

    In a perfect world, the location of the restriction would not depend on where you put it. The same orifice will create the same pressure drop with the same flow. That's in theory. In practice I would shy away from putting anything in the suction side.

    For either type of pump, unless you really need that pressure drop before the inlet of your pump, I would not put a restriction on the suction line of a pump, especially on a centrifugal. Your aim in the suction line is to provide as even a flow as possible. A restriction or even bends and fittings will cause disturbances that will effect the efficiency of the pump. Without knowing excatly what it is you are doing by putting the restriction in the suction side, I would just say don't do it. If you are trying to limit or control the volumetric flow from the pump a control valve or regulator/relief may be a better option. Again, I don't know what you are trying to accomplish here.

    As far as pump type, IMO I would think that that would depend, first, on desired flow rate. If your flow is high, you may not be able to find an economical PD pump. I prefer to use PDs myself.

    If you can, let us know what your end design is doing and more details and maybe we can go further with this.
  6. Jan 11, 2005 #5


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    If I have understood you well, you want to put a restriction either at the pump inlet or at the pump outlet.

    Sure the loose of stagnation pressure (i.e. mechanical energy) are different in both cases. I think the efficiency of the pump will be approximately the same, if you consider only the pump and not the restriction upstream of downstream. The efficiency of the whole could vary indeed. The losses of stagnation pressure are dependent on which kind of flow you have, on Reynolds Number and on the flow velocity explicitly. Try to get a loose coefficient for your particular design. And be aware that putting the restriction before the pump will distort the inlet pump flow, maybe this could enhance a worse flow perform at the blades entrance and affect to efficiency.

    I don't know if this could help you. :frown:
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