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View Full Version : Can you compute overflow water level height?


bbrantley
07/05/2006, 03:32 PM
Does anyone have a formula, or an intuition for a formula, for computing the height of the water above an overflow given a specific linear overflow length and flow rate? I realize that it's an arbitrary fluid-flow problem when you can have teeth of different thickness, because friction with the teeth starts to dominate. Perhaps there are other parameters that affect it to first order as well. But in my case, I would like to have a continuous overflow with no teeth. I'm just trying to figure out roughly how wide to specify it for the tank I'm buying.

Thanks for your help!

Ben

Mutagen
07/05/2006, 04:11 PM
For V notch
Q = 1140 * H^5/2 - H is in feet and is measured from bottom of V to top of liquid level. Q is in GPM. Equation applies only to a 90 degree V.

For rectangular notch
Q = 1495 * H^3/2 * (B-0.2H) - Again H is in feet and Q is flow in GPM. B is the length across the wier.

For mutiple notches multiply by the number of notches used. These equations are closer estimates when talking about larger weirs, but at least its a start.

BeanAnimal
07/05/2006, 04:34 PM
yup standard old sharp crested weir equations can be applied with some success.

Ben make it the entire length of the tank. That way you will have superb surface skimming. I put at least 1500GPH over a 4' internal weir. The water is less than 1/16" deep. Again you can use standard weir equations to determine the depth of a given amount of water that will flow over a weir of width X.

Bean

bbrantley
07/05/2006, 09:49 PM
Thank you both. Mutagen, what are units on that constant? Or, alternatively, what are the units of b?

Bean, I appreciate the advice. I will have two sides to choose from; one will be over 8 feet and the other nearly 5. Things impacting my analysis include:

1. The height of the water over the overflow, of course. I think too small may actually be disadvantageous. I want it to be able to pull floating sludge off the top, in other words.

2. #1 again, but for evaporative reasons. Given sufficient skimming potential, I'd rather not overkill simply because it'll drive up the humidity in the room and make my heaters work harder. (Cold climate.)

3. I'm going to have a hang-on-back overflow box installed. This is slightly cheaper to do for shorter distances, but no matter which side it's on it will impact my ability to reach into the tank from the edges. With this size tank and the in-wall constraints that I'll have, I expect to be on top of the tank a lot of the time anyway... but every little bit helps.

Given those, I'd love to hear your further thoughts.

I ran the numbers in your example and came up with something closer to 1" over the 4' weir, but I may have the units wrong, I may have screwed it up, or it may just be inaccurate for these extremum.

Thanks for your help,

Ben

BeanAnimal
07/06/2006, 06:59 AM
How many GPH are you planning on pushing over this thing?

IS drilling the tank and building an internal shelf out of the question?

bbrantley
07/06/2006, 10:15 AM
<a href=showthread.php?s=&postid=7689424#post7689424 target=_blank>Originally posted</a> by BeanAnimal
How many GPH are you planning on pushing over this thing?

IS drilling the tank and building an internal shelf out of the question?

Very few GPH, as little as I can get away with. Don't need a lot of flow to skim and feed a refugium. An upper limit of 2000 GPH, but I'm shooting for about 1000-1200 as a first try.

Nothing's out of the question yet since the tank hasn't been built, but I'm wary of an internal shelf because of aesthetics and ease of cleaning. (I presume by "internal shelf" you mean a box glued to the top of the tank that only extends down a few inches, similar to what I'd have hanging off the back but inside instead?) It's an interesting idea, though -- certainly one I hadn't considered.

Ben

BeanAnimal
07/06/2006, 10:40 AM
That is exactly what I mean. It will give you superb surface skimming and with the size of tank you are describing, it should be very non-invasive.

Bean

bbrantley
07/07/2006, 10:34 PM
<a href=showthread.php?s=&postid=7690482#post7690482 target=_blank>Originally posted</a> by BeanAnimal
That is exactly what I mean. It will give you superb surface skimming and with the size of tank you are describing, it should be very non-invasive.

Bean

Thanks Bean, I'll take that into consideration. It seems to me the drawback with that approach is that I have to reach around it to scrape the side below it. If there's one thing I hate most, it's scraping. :eek2:

Ben

BeanAnimal
07/08/2006, 07:11 AM
I don't scrape the back of my tank :)