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Venturi effect witout/with nozzle

  1. Apr 6, 2015 #1
    Hi,

    I am building a hose system with an automatic liquid dispenser using venturi effect.

    The 12v pump is taking water in a tank via a 3/4 pipe and has a capability of 2.9gpm at 50psi. The main system is built with 3/4 pipes and a 5/8 hose. The siphon pipe is 1/4 and I added a check valve to make sure the water will never go in my liquid tank.

    I made an image with the schema of my current prototype.

    pump-system.jpg

    Everything is working perfectly until I put a nozzle on my hose. When the nozzle is closed, the check valve automatically closes, witch is normal and what I desire.

    The problem is, when I open the nozzle, the check valve stay closed. The hole off the nozzle is smaller than the hose to have more pressure. This is probably the cause of the problem.

    What am I missing? What can I do to make sure the venturi valve is still working with a nozzle?

    Thanks,

    J.
     
  2. jcsd
  3. Apr 11, 2015 #2
    I would be curious as to the flow rates in the two configurations, a test easy to do. Get a one gallon jug and time how long it takes to fill the jug with nozzle on and opened a typical amount and also time how long to fill the jug with no nozzle.

    The difference in flow rates translates to different pressures developed at the venturi. At low flow rates the pressure difference might not be great enough to overcome the pressure of the check valve? Possible solutions, increase venturi effect or reduce the force of the spring in the check valve, or easyier increase the gravitational head of the tank to counter-act the check valve spring force (actually that will only buy you a psi per 2 feet of head)?
     
    Last edited: Apr 11, 2015
  4. Apr 11, 2015 #3

    sophiecentaur

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    Yes. The venturi operates on volume flow. This pressure difference may not be very much, of course (thinking in terms of car carburettors, which work on a few mm of head ) But the check valve is measuring the difference between the tank feed pressure and the pressure in the delivery pipe (not the venturi pressure difference). If you use a nozzle, this can be too great for the venturi (-) pressure to cancel it. One way round this could be to raise the tank a bit and increase the feed pressure (which is just not enough to push liquid through the check valve).
    I think that this 'passive' control arrangement is probably a bit knife-edge so you may need to fool around with heights and nozzle sizes to get things as you want them.
    One solution, (more complicated, of course) could be an electrical pump / pressure switch arrangement.
    Another solution is to do what car washers often do and have the detergent feed within the nozzle where the pressure is much lower. (built in venture effect)
     
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