Tank Design Help

[sphelps]

Originally Posted by Canadian

As a real world example: the overflow in my AIO is 4" long with a flow rate of around 225 GPH. A 3" overflow is adequate to handle that flow rate but even at 4" the surface of the water is around 3/8" above the overflow. The "velocity" of flow is great enough that it "draws" things towards it but the majority of the actual water flowing over the overflow is coming from beneath the water's surface and is therefore woefully inefficient. If I adjust the flow so that I don't get any surface agitation I can see the dust and organics accumulating on the water surface and how little of it is actually drawn over the overflow.

I find similar results with my tank. The overflow has about 10" of linear distance and at my standard flow rate of about 1200GPH it works great but if I reduce the flow same deal, I get a scum build up. Your theory would seem to suggest you need to increase the overflow size for the smaller flow rate, which to me doesn't make any sense. In fact when I cover part of the overflow at the lower flow rate I can immediately see the surface scum being pulled towards to overflow. Don't get me wrong the theory is sound the larger overflow will pull water from the surface but you need a larger flow rate to create the surface tension so it draws in water from across the tank, not just the area around the overflow.

In the end I guess we have to all do whatever makes the most sense to us. But I would have to disagree and say it's much easier to make an overflow to big than too small. Worst case if the overflow is a little small you'll bring in some more water from below the surface, this doesn't sound like a bad effect as the increased surface tension will still skim the surface efficiently, if the overflow is too big for the flow rate you don't get good surface skim, it's right there in your own example.

[StirCrazy]

I am going to side with Canadian on this one, a single overflow will be more efficient, but in the design above it won't. for a single overflow to outperform two it has to be larger, this is why the coast to coast works so good. I would recommend going with one large one in the middle, maybe 24" wide by 3" deep. or how ever deep enough you need to get your bulkheads in. If you can go even wider.

Also we shoot ourselves in the foot when we make overflows because we put teeth in them. this allows water flow from below the surface to enter the overflow.

My last tank I think I did about 5 different overflow designs over the years and the one I was most happy with was an acrylic overflow box with no teeth, just a smoothed out rounded edge. this combined with a large linear distance will cause a very thin film of water to flow over the edge. I think in my 94 I had 1500gph overflowing at a water thickness of less than 1 mm.

Steve

[Parker]

I guess we could all agree that there is more then one way to do things, I just have to sit down go over all of my options and come to a decision on what works best for my situation.

I don't have much room behind the tank so the less pluming I have to go over top the better. There was a screw up when the cabinetry was installed and the tank ended up being 4" closer to the wall then I wanted.

I'm not sure a center over flow would work. I believe the bracing on my stand might get in the way, and to modify the stand I need to remove the countertops, crown & riser , hutches plus a stub wall in the basement to get the stand out to the garage. I'm not grinding and welding in the basement!

Ahh screw it, where's my 10 galon tank I'll set that up!

[Canadian]

Originally Posted by StirCrazy

I am going to side with sphelps on this one, a single overflow will be more efficient, but in the design above it won't. for a single overflow to outperform two it has to be larger, this is why the coast to coast works so good. I would recommend going with one large one in the middle, maybe 24" wide by 3" deep. or how ever deep enough you need to get your bulkheads in. If you can go even wider.

Also we shoot ourselves in the foot when we make overflows because we put teeth in them. this allows water flow from below the surface to enter the overflow.

My last tank I think I did about 5 different overflow designs over the years and the one I was most happy with was an acrylic overflow box with no teeth, just a smoothed out rounded edge. this combined with a large linear distance will cause a very thin film of water to flow over the edge. I think in my 94 I had 1500gph overflowing at a water thickness of less than 1 mm.

Steve

I don't know what part of sphelp's points you agree with Steve when you say:

for a single overflow to outperform two it has to be larger, this is why the coast to coast works so good

because sphelps is contending that a larger overflow would decrease the suction of the overflow and make it less efficient.

[StirCrazy]

Originally Posted by Canadian

I don't know what part of sphelp's points you agree with Steve, but



sphelps is contending that a larger overflow would decrease the suction of the overflow and make it less efficient.

sorry, typed the wrong name LOL

[mark]

could easily build a Coast to Coast internal, and drain through the bottom of the tank (OF Tee shaped, think 2x4 on the flat sitting on a 4x4 post, or even the vertical box to one side, with your standpipe, assuming Herbie, in the 4x4 section).

RC has a calculator that gives weir length for flow. Not sure the reasoning but will give a water height of about 1/4" above the weir with the rated flow.

[superduperwesman]

Generally the extreme of any case proves the point.

Canadian your right that you don't want a overflow that's too small. If I have a flow rate of 3000 gph and a 3" linear overflow I'll be sucking a lot of water from under the surface and potentially not getting a lot of surface skimming.

sphelps you're also right that you don't want and overflow that's too big. If I have a flow rate of 1 gph and a 10" linear overflow the surface of the tank will get build up like crazy.

To small or too big can be bad but like sphelps said I'd rather error on the side of being too small and sucking out water from below the surface then have one that's too big and have some nasty surface buildup.

It all depends on how much flow you have coming from the sump. Some people prefer power heads for the majority of flow so the sump return pump is pretty small in which case you wouldn't want a really big overflow.

[Parker]

Thanks for all the adivce and the lessons in fluid dynamics I think we have the design wrapped up.

[sphelps]

Originally Posted by Canadian

because sphelps is contending that a larger overflow would decrease the suction of the overflow and make it less efficient.

Not necessarily, I'm saying the size of the overflow is more dependent on the flow rate than anything else and that bigger isn't always better. Practical experience and results, including your own example, shows overflows work better when properly sized and if the overflow is too large for the flow rate you may end up with surface scum build up.

With a longer rectangular tank i think you're better off with a single overflow on one side and return on the opposite. This way you're pushing the general flow towards the overflow and not away from it and filtered water has to travel the full length of the tank before it can be skimmed off again. It just makes more sense to me.

[Canadian]

No, in my example I would contend that while 3" is adequately sized to accommodate the flow rate, even at 4" the majority of the water flowing over the overflow is coming from beneath the water's surface and I would benefit from an even longer overflow to improve the efficiency of protein-laden water being drawn off the surface.