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Help calculating water flow into an overflow

  1. May 27, 2009 #1
    I have an acrylic tank that has an overflow area on each end. The tank has a top on it that is seamed in place. There is a 1/4" gap between the top of the overflow wall and the ceiling of the tank AND there are 42 - 1/4" x 1" cutouts along the top of each overflow wall. So, the question is, how do I calculate the amount of water that can flow through these spaces and into the overflow areas in a given period of time? Surely there is a magic formula somewhere that will help with this calculation! I have attached a drawing of one overflow as a pdf. There is one at each end of the tank. Also, I assume that the fact that this is salt water will make a difference to the flow rate and thus the volume per hour. Why, you ask, do I need to know this? Because I am designing a closed loop circulation system on this tank and don't want to push more water through the circulation system than can actually fit. There are cut outs in the top which would allow salt water to flow into my basement and run the tank dry if I goof this up. Thanks for your help with this. i know corals and fish, but am short on fluid dynamics!
     

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  2. jcsd
  3. May 28, 2009 #2

    nvn

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    Are the cutout slots vertical, so that their length is 25.4 mm in the vertical direction? For these cutouts, what is the acrylic pane thickness? And what is the tank depth? Does the 6.35 mm gap along the tank ceiling cover a total length of 1270 mm, including both ends of your tank?
     
  4. May 29, 2009 #3
    The pdf called "overflow" is a top view of the overflow area. The "tankside" one is the whole aquarium from the side. The one labeled "tanktop" is what the tank looks like for the top with the overflows on the ends.

    The water SHOULD always be lower than where the top of the overflow walls are and therefore the entire cutouts will not be underwater, but it is important to know how much water CAN flow through the cutouts in a given period of time. The return flow tubes will be powered by the pump(s) and should match the volume that the overflow can handle. Basically, the water pumped into the tank to cause the overflow of water through the cutouts will come from the return flow pipes. I will add as many of these as are needed to generate the most flow the cutouts will allow.

    I will have ((42 x (.25" x 1"))*2) square inches of area for the water to flow through without flowing OVER the overflow walls - something we want to avoid because it removes the margin for error - ie water splashing out onto the floor! Does it matter that it is salt water instead of regular water? It has a higher specific gravity... harder to push, right?

    The slots are vertical. The acrylic wall the slots are in is 1/4 inch thick. The tank height is 24" - externally, not sure internally. Does this info clarify enough for you to figure this out for me?
     

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  5. May 30, 2009 #4

    nvn

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    I currently don't think salinity has a major effect on the flow rate. If the tank water level is exactly 25.4 mm above the bottom of the cutouts, I currently think the total flow rate, out of 84 cutouts, will be 160 liters/minute. It would be good if you could test the flow rate out of one cutout, using the same cutout dimensions and wall thickness, and a container several times deeper than the vertical dimension of the cutout. Adjust the input flow rate to obtain a steady-state water level 25.4 mm above the bottom of the cutout; then catch the input or output for 300 seconds, and measure the number of milliliters, then divide by 5000 to obtain liters/minute for one cutout.
     
    Last edited: May 30, 2009
  6. May 30, 2009 #5
    Thanks! I will try to run a test, but it is going to be tough to arrange. I will make adjustments in place, so having an initial estimate is VERY helpful!
     
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