Tank Design Help
 

All,

I'm looking for some feed back on my tank design before I submit it to the manufacturer. Comments / Concerns?


http://i200.photobucket.com/albums/a...ankDrawing.jpg

dims?

Sorry about that: 84"x24"x30" High

Have you considered external overflows? I currently have one, and LOVE the fact that it doesn't take any space in the tank (and it looks better, IME).

But of the two choices you've given, I'd pick #2, simply because it would provide more surface skimming.

I can't do externals, the new one is replacing this one, would have been nice though.


http://i200.photobucket.com/albums/a...d/IMG_1990.jpg

Agree with Drew. Dual overflows on the back of the tank would be fantastic.

Also with #1 if you ever have to fish something out of the overflow or need to put your hand in for any reason, it could be a right royal pita.

Good point, didn't think about having to get my fat arms in there..

Oh trust me you will have to at some point... :mrgreen:

I vote for 'B', the overflows would take up a bit more space but would be more efficient and easier to work with. My tank has a triangular overflow and it's pretty tight. I also assume there is eurobracing not shown in the picture, the eurobracing on my tank makes the overflow even tighter.

JMO

Do you need two overflows? I know it seems to be common practice to run two on larger tanks and correct me if I'm wrong, but wouldn't you get better surface skimming with one overflow? Reasoning for this is that a single overflow alone will bring in twice the water as an overflow in a dual setup. This will result in a higher water velocity on the top surface and therefore greater surface tension which to me would suggest better surface skimming.

In addition you would run your return on the opposite side to create a general flow pattern towards the overflow which should produce a more efficient turnover. Plus you have more defined upstream and downstream areas which can be beneficial when keeping different types of corals.

Single overflow boxes have been common practice for the tanks I build and I've always had good results. More cubic tanks are a little different but for the longer narrow tanks a single overflow and return on the opposite just makes sense to me.

The greater the linear dimension of surface skimming you have the better you will be able to get laminar flow over the weir as opposed to pulling from beneath the surface for a given sump return pump flow rate (assuming your plumbing can accomodate the desired flow rate). More laminar flow over the weir ensures you're preferentially pulling the proteinacious film off the surface of the water.

So go with B and definitely don't go with one overflow unless it's coast-to-coast (or near cost-to-coast).

I don't know how much room, if any, you have behind the tank but take a look at this site for a good overflow design (it's also a good site for others to get a handle on how to improve efficiency and safety in their overflow design):

http://www.beananimal.com/projects/s...ow-system.aspx

There is a thread in the DIY section of RC with more pictures and discussion if you're interested.

I know what you're getting at but a single overflow still won't produce turbulent flow, it will still be laminar so a higher velocity would still be better. Ideally you would want the smallest "linear dimension" without entering the transition or turbulent flow regime, a Reynolds number less than 2000. A lower velocity will result in lower surface tension, and could result in surface build up in some areas. Kind of like a vacuum cleaner, a large area results in low velocity and it can't bring in particles at a distance, decrease the area and the suction is magnified and you can bring in particles from larger distances.

The main point here is why waist the space with two overflows when one is good, possibly even better. Same idea as pipeline design, you want laminar flow but why use 12" when 6" works? Bigger isn't always better.

In the case of a weir I was under the impression the idea was to primarily increase the water level of the river upstream. The reason the overflow length is so long, usually covering the full river width, is to minimize water level fluctuations upstream due to inconsistent flow rates. Never heard of anything relating to surface skimming, could be wrong though.

A longer overflow (read: larger linear dimension of the overflow) will allow greater velocity of surface skimming and more efficient removal of surface-bound proteins. Instead of pulling non-protein laden water from below the surface with a small overflow you maximize the removal from the surface with a longer overflow. Ultimately the flow rate could theoretically be the same for both the shorter and longer overflow but the longer one pulls more preferentially from the surface as opposed to the shorter one pulling water from under the surface. Therefore the shorter one could have the same flow rate but actually take considerably longer to remove the surface-bound proteins rendering the shorter one being considerably less efficient and unnecessarily wasting energy consumed by the return pump (assuming you're trying to match your overflow rate to your protein skimmer flow rate to maximize efficiency and not using your return pump to add flow to the tank).

This the basic principle behind the "Calfo style overflow" which greatly enhances surface skimming. Take a look at the link in my previous post.

I'm aware of the calfo overflow design and I've used it before but didn't see the results of better surface skimming. Increasing the linear distance doesn't increase velocity, how could it since you're increase the area, so how can it increase surface skimming? The link doesn't provide any real information on larger overflows, it seems to be based more on a silent standpipe design.

It doesn't increase the velocity of water flowing over the overflow, it increases the velocity at which surface-bound proteins are removed because only the surface water travels over the overflow instead of half of the flowing water coming from below the surface.

There's a rather succinct example in the link I provided:

I think if I understand it correctly ( which I probably don't ) Canadian is saying that a smaller over flow will act as a siphon of sorts and pull water not only from the surface but from further down in the water column depending on the speed of the plumbing and return pump?

That probably makes no sense....

Yeap I know what you're getting at but again like I said before theirs obviously a limit to how small the overflow can be. If you compare a 40' overflow to a 4' overflow of course the 40' will work better since water level will be significantly higher above the overflow. What I'm saying is a 20' could clear the oil faster than a 40' depending on flow rate.

Your theory is sound but I don't think the biggest overflow is necessarily the best. If the overflow is too big for the water flow insufficient surface tension could result. I've seen this before. The way I see it is the second overflow is not needed so why waist the space?

Yeah that's the basic idea, if the overflow is too small water will rise above it to the point the majority of the water is taken from below the surface. However I don't think we run that kind of flow rate in our tanks to create such an effect with a single overflow. There's a limit to how small you can make it as well as a limit on how big, however for simplicity I can see how the bigger is always better principal is often applied not only to this but other applications as well. I'm just saying this isn't necessarily that accurate.

From a practical standpoint in an aquarium I don't see it being feasible to create an overflow that it is so large that it creates insufficient surface tension at the typical flow rates used in aquariums with typical dimensions. I do however see overflows that are too small resulting in the majority of the water being drawn in from beneath the surface. You'd need to have a ridiculously long overflow at an incredibly low flow rate.

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.

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

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! :mrgreen:

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

because 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

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.

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.

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.

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.

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