Tank Design Help

[Canadian]

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.

[sphelps]

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.

[Canadian]

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.

[sphelps]

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.

[Canadian]

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:

Surface Renewal

An extreme example: Take two identical 40' wide ponds that are fed with the same amount of water, one with a 4' wide dam and the other with a 40' wide dam. It follows that for the same flow (lets say 100 gallons per minute) over the dam, that the narrow dam will have a very thick waterfall and the wide dam a very thin sheet of water cascading over it. Now place an oil slick over both ponds! It will take significantly longer for the 4' wide dam to clear the slick fro the pond. Why? Because much of the water flowing over the dam is from below the surface! Now apply this logic to your tank, but instead of an oil slick, understand that the surface of the water attracts organics from the tank. The logical conclusion? For any given flow rate, the wider the overflow, the better your skimmer and/or in sump filtration will work!

[Parker]

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....

[sphelps]

Originally Posted by Canadian

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:

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?

[sphelps]

Originally Posted by Parker

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....

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.

[Canadian]

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.

[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.