How is linear overflow calculated?

[hmott]

I thought I knew, now I'm not so sure.

If I have a 10" by 10" box in my tank that will then flow over some U tubes. I know the U tubes need calculated and that I can do easy. But you also need to calculate "linear overflow".

Is Linear overflow like area? 10"x10" = 100 inches of linear overflow?

Is it like perimeter? 10x3sides = 30 inches of linear overflow?

Is it neither?

I assumed it was the perimeter but I'm not so sure now. Several of the commerical overflows are way smaller than whats needed for linear overflow area and their claim of gph if thats the case.

[BeanAnimal]

30 inches

The linear overflow distance has nothing to do with capacity. The longer the linear overflow distance, the better the surface skimming... thats all.

Bean

[Avast Marine]

So the same 10x10 box with 10 1" long 1/2 slots has a linear overflow of 30 + ?

[BeanAnimal]

No the slots reduce the amount of overflow by sum of their widths... Assuming the slots are 1/2" teeth, with 1/2" spaces in between, then the linear overflow is about 15 inches.

[hmott]

well the calculator on the homepage here at reefcentral certainly does include the minimum requirement for linear overflow in the calculations of flow rate for overflows. So it seems they don't believe its unrelated to flow and only useful for skimming.

Whats the deal? I never considered the possibility of the overflow length/width being a bottleneck but it seems its possible.

[BeanAnimal]

It's just a minimum recomendation for surface skimming... thats all.

The more water you put over a "linear" overflow, the deeper it becomes. The deeper it becomes the more of a waterfall effect you get. This has nothing to do with the flow rate of the standpipe or overflow. The bottleneck is the standpipe diameter (among other things).

Bean

[Jandree22]

Assuming I exactly match the linear width per recommended by the calculator, what kind of rate and depth over the threshold is the water going to flow... more of a gentle trickle or a healthy waterfall?

[BeanAnimal]

If you use the minimum number as provided by the calculator, you will get a moderate flow, maybe 1/4" thick or so. For example, it calls for 30" for my 2000 GPH overlow. I have 48" and the water is less than 1/4" thick and silent (no tubulance, it sheets over).

[Jandree22]

Perfect... just the sort of answer I was looking for, thanks :)