Flow Eductors - any use in a planted tank?

Discussion in 'Filters, Filtration and Pumps' started by Ray, 26 Aug 2008.

  1. Ray

    Ray Member

    31 Oct 2007
    This Ebay seller (http://stores.ebay.co.uk/International-Fish-Street) is offering Flow Eductors (http://stores.ebay.co.uk/Internatio...Z4QQdirZ1QQfsubZ12840653QQftidZ2QQlnsZ1QQtZkm):


    The linked review (http://www.advancedaquarist.com/issues/sept2003/review.htm) says:

    Would this help with flow in a planted tank and is the outflow from an external filter "pressure rated"?
  2. aaronnorth

    aaronnorth Member

    19 Feb 2008
    worksop, nottinghamshire
    In theory yes, it is similar to a lily pipe.
  3. Ed Seeley

    Ed Seeley Member

    3 Jul 2007
    Bear in mind these have no moving bits and have to obey the laws of physics so while it may move a bit more water it does so at a lower velocity so may not have quite the same punch. The restriction of flow will also increase the frictional head on the filter/pump and reduce the flow rate slightly.

    Having said all that they may well have the desired effect in a smaller tank of moving more water.
  4. Steve Smith

    Steve Smith Member

    19 Jul 2007
    Leamington Spa, UK.
    Anyone studied hydrodynamics? :D

    Interesting idea, but ugly!
  5. ceg4048

    ceg4048 Expert/Global Moderator Staff Member

    11 Jul 2007
    Chicago, USA
    Yeah, I agree with Ed. It's a good idea but you don't get something for nothing. The mass flow rate increase is an illusion because you are accelerating the low velocity water in the vicinity of the cone to mix with and add to what is being ejected from the filter. I would rather have the higher velocity, lower mass flow rate to penetrate into the far reaches of the tank. The actual filter (or pump) turnover is more or less unaffected. The reviewer makes it sound as if somehow you magically get 245% more turnover from the pump. What you are doing is using the energy of the effluent to move nearby water that has not yet been processed by the filter, however this itself might be great if it generates better circulation patterns in the more vulnerable areas of the tank.

    Also the water does not "expand". The venturi effect creates lower static pressures at high fluid velocities inside the cone. This lower static pressures then draws nearby water (which is at a higher static pressure) into the cone. This is how your python water changer works. Due to frictional forces, the velocity near the cone's inner surface is no doubt lower than at the center of the cross section. As the two waters mix there will be turbulence which robs total energy from the flow so the assumption of constant velocity across the section is probably not 100% accurate.


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