High flow VS low flow returns

[kpiotrowski]

Please remember that it is FPS and not CFM (or GPM) that keeps detritus in suspension.

Or, without the abbreviations, it is velocity (feet per second) and not flow (cubic feet per minute or gallons per minute) that keeps detritus suspended.

So as long as the thickness of the flow layer is larger than the suspended particle (you can't move 2-mm diameter Urchin pellets over a toothless coast-to-coast if the flow is only 0.5mm deep), you need to compare the water velocities going over the overflow to determine which one would be more effective for detritus.

So convert the flow to CFM (about 7.5 gallons per cubic foot), and divide by the overflow area (length times depth of each section or "tooth space" times the number of tooth-spaces) to get velocity.

yes that is the question. what is that Ideal thickness?

 

[dnjan]

There are equations for calculating the velocity necessary to keep a particle suspended if you know the specific gravity of the particle and the particle size.

The specific gravity of sand is about 2.6, but I don't have a guess on the specific gravity of detritus.

If you assume a specific gravity of 2.0 and a detritus particle size of 1/32 inch, the velocity is a bit over 7 inches per second.

 

[kpiotrowski]

yeh way too many variables there like with sg of sand taking your word of 2.0 but then again what is that sand composed of, what materials are attached to it and along for the ride etc? so safe to say nobody is going to give anyone a grant for 100k to study this, however dnjan being the math guy, what would you recomend as a happy medium. could you give a ballpark on length of oveflow considering size of display,and how high of a wall of water should flow over that overflow?

 

[dnjan]

So, using an example from much earlier in the thread of an overflow of 1,000 gallons per hour, and a 72" toothless coast-to-coast overflow,
That calculates out to a flow thickness of about an eighth of an inch for a 1/32" detritus particle having a specific gravity of 2.0

Note - with flow rate (gph) being constant, if your water thickness going over the overflow is larger, the velocity is lower. So too thick a layer would be moving too slowly to carry the detritus with it. Seems counter-intuitive at first, until you think about it a bit.

I don't know how to calculate (actually can't remember - supposedly I knew at one time ...) how thick the flow layer would actually be for 1,000 gph over a 72" toothless overflow. I just know that if that is really the flow rate, then if the layer is thicker than an eighth of an inch the velocity will be too low to carry with it the above-assumed detritus particle.


I did a few quick calculations for my own tank, which has a couple of long-spine sea urchins. Their detritus particles are so large that the layer thickness that gives enough velocity to keep the particles in suspension is smaller than the particle size. No wonder I collect so much urchin-$#!% in my tank!

 

[kpiotrowski]

"velocity" you can shoot crap as fast as you want, but if it never reaches that sweet spot at the overflow where velocity would have it sucked in and carried over the wall..... man we try too hard sometimes huh!

I guess I will stick my "sayin" "move the heck out of the water in the DT and then move it the heck out" and call it a day.

 

[dnjan]

... and occaisionally siphon the display tank.