DIY Overflow

[smithythe]

So I plan on building an overflow box for my 36"x12"x12" shallow tank. I have studied various overflow boxes and need some advice on working out the size I need to make it. I plan on constructing this with 3mm clear acrylic sheet. As a start point I have made it 200mm long with a 2000mm cross sectional area (10mm gap).

Aquarium Approx. Volume 90L
Anticipated flow rate 900l/h

I have the following basic sketch of the box:
View attachment 3418

I need some advice on cross sectional area through the box and the size of the outlets on the filter side. Also do I need the comb weir plate higher, lower or level with the outlet weir plate?

Any advice or previous experience would be appreciated.

 

[smithythe]

I have been doing some more research/thinking about this and I guess a good place to start would be to work out the length of the overflow box based on how much water will pass through the comb.
I have chosen some aribitary values to get started:
Height of comb: 10mm
Width of gaps in comb: 3mm (not sure if this is optimum as I guess this needs to be big enough to get as much flow rate but small enough to stop critters and large pieces of debris getting sucked in)
Flow rate: 900l/h (10x aquarium volume)
Width between gaps in comb: 3mm
Planned width of overflow box: 200mm (as a start point I dont want it to be much bigger for asthetic reasons)

Ok numbers - I found this site http://www.fhwa.dot.gov/engineering/hydraulics/pubs/06086/hec14ch11.cfm about grate design. Altough this is for calculating the length of grate (comb in this case) I think it would be safe to assume that the calculated length will approximate to the width of the comb required as I am only calculating one dimension (10mm fixed height or width in this case). Also this equation is based on water flowing vertically but all I am doing is swapping the entry and exit orientation of the grate, so I guess this is ok?!

Using this formula:
View attachment 3419
where,
C = experimental coefficient equal to = 0.245
W = width of the slots, m = 0.003m
N = number of slots (spaces) between beams = 33 (assume that comb is 200mm long, slots are 3mm and gaps between slots are 3mm, =200/(3+3) ~ 33)
yo = approach flow depth, m = .01m
Q = flow rate = 2.5e-4m/s (900l/h = 0.00025 cubic meters per second)
g = gravity = 9.81m/s^2

This calculates to 0.023m. Thus if I increase the comb length to 250mm, I will also increase the number of gaps and improve flow giving me a bit of overhead.

So the total comb length(overflow box) will be 250mm long and 10mm high gaps, 3mm wide at 6mm centres giving me a flow rate through in excess of the target 900l/h.

I plan on adding a large sump to this set up will I need to factor this in when calculating turn over rate. i.e. if I have a 90l aquarium with a 90l sump and go for x10 turnover rate will that equal 1800l/h or 900l/h?

This is all based on rough guess work and a lot of assumptions, if anyone can spot any obvious errors or tell me I am barking up the wrong tree altogether please let me know!

 

[Tom Barr]

You might consider a simpler to make design.
And a venturi type bubble remover to prevent siphon breakage.

Check out Mame's overflow(do a google search and then watch the video.)

 

[Spiracle]

Dude, I'm glad your into physics, but don't try to reinvent the wheel. I made a CPR replica about a year and a half ago using the CPR dimension guide. I have a custom built tank with glass euro brace so I had to make a custom overflow. View attachment 3897View attachment 3898

Dimensions found here:
Marine depot.com/overflow_boxes_cor-ap.html

Let me know if you have any questions. It's not that hard to make.