Outflow depth calculation for overflow box?

[CThompson]

Is there such a thing as a calculation to work out the depth required for the outflow section of an overflow box?

Something like;

Overflow entrance width (in centimetres) multiplied by the pump litres per hour = overflow depth.

(Width by LPH = depth)

The overflow box will be attached at the end of the tank, and I am planing on a tank width of 66cm (622mm minus glass thickness of 2 by 19mm) and a pump of around 10,000 LPH.

I haven’t made a final decision for the pump as yet, so the total LPH may vary, so if there is some sort of calculation known by one of the resident gurus, it would be appreciated.

I need to know how much lower this end of the tank must be than the other end of the tank?

Obviously a cm or two would be added to the nett calculation

Thanks,

Craig

[Guest Gutty]

Have only glanced over it and didn't see the info you were after but "might" be somthing here

[CThompson]

Thanks, but it didn't help.

Craig

[Cichlids_au]

I'm not exactly sure what you’re after here Craig. Is it the flow rate from the tank to the overflow or the flow rate from the overflow to the pipes to the filter?

You see the more important calculation is the rate the pipes clear the water from the overflow to the filter. The rate the water flows from the tank to the overflow is less important than that is.

You will also need to consider the rate of exhaustion of the prefilter media as that will also cause an effect on the clearance of water from the overflow.

[CThompson]

Mark,

If I have an overflow box on the end of the tank, the end glass of the tank that goes into the overflow box must be lower than the actual height of the tank, so the water flows out of the tank here, and into it the overflow box.

If I for example organise this end to be 2mm lower, when I turn the pump on (pump moving 10,000L/ph), the 2mm drop, will not handle all the water the pump is putting out. And will probably also come out/spill at the next lowest point (ie all over the sides of the tank and into my house).

In this example, there would be enough water in the sump to overflow the tank, if the overflow outlet is not low enough to handle the volume put out by the pump.

In the overflow box, there will be two standpipes, and wether they are 20mm high or 300mm high there will be enough water in the sump were the pump is, to overwhelm the overflow outlet, because the tank outlet is being overwhelmed not the overflow box itself.

I understand your comment on the filter wool getting dirty (to put it in layman’s terms), and yes, this can be a factor for the rate of escape of the water from the overflow box to the sump. But once again, before this happens, the tank itself will be overwhelmed. The water has to get into the overflow box first.

The issue is water getting into the overflow box fast enough to cope with the pump volume. Not the water getting from the overflow box to the sump.

The return water will be going along a 40mm pipe, if I had a return pipe out of the tank of say 30mm (anything less than 40/50mm), well obviously the outlet will be overwhelmed, and the extra water from the sump will be pumped up, and the tank will overflow.

In my case, I will have an inlet pipe to the tank of 40mm, but the water won’t be going out to the overflow box via a pipe, but by the open end (lower end) of the tank. In the overflow box, I intend to put in two standpipes, to take out any possible room for the outlets to be overwhelmed. And in addition, I am intending to put in a third “emergency” standpipe that will just vent, bypassing all media, into the body of the sump itself. My mate Murphy reckons that’s a good idea.

So, I guess my problem is, how to equate the difference with the 40mm return into the tank, with the width of the end drop of the tank and unknown depth, with the volume of water produced by the pump.

I imagine that if I left a 30mm drop (lower end) this will take care of it, but I would like to know a calculation so that I can allow accurately, then add some.

Craig

[Guest Gutty]

I would think double the outlet surface area should give you enough safety factor Craig.(we use a similar system on one of our EDM machines here at work, and thats what we worked on, though i have no idea what literage we are actually pumping)Not being a fluid engineer though, better get some more specific info

The demo at the bottom of this page may be of some help. let me know if you need any CAD files to go with it.

Also might be something useful here.

Other than that i reckon it'd be a pretty simple equasion for some one trained in fluid dynamics. Maybe try emailing a couple of universities ??

Again i haven't had time to thoroughly go through those links, they "might" be helpful.

Matt

[Cichlids_au]

OK Craig I get what your after. I'll try to get a calaulaton for you.