I've read the stickies for diy overflows for days now but not much has been said about the shorter tubes. The fact that there is two overflows on the same fixture won't up the gph? I do like the siphon idea but I wouldn't like loosing water from my tank. If I do use the siphon method, how can prevent air from breaking siphon without using a water pump?
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There may be some in disagreement but this is what I've found over the years with hydroponic, aquaculture and agriculture projects and explained with my experience in physics.
The short version:
Longer siphon tubes work better. By increasing the amount of water held vertically the internal resistance of the horizontal sections (as well as loss due to direction change) starts to become negligible. Short tubes have less water mass held in equilibrium so the horizontal sections have a larger effect.
Your discharge math based on the point of peak restriction and how fluids moving in different directions interact. Since you are dealing with pressure differentials any resistance can have a compounding impact on overall efficiency, especially when you start getting water to move fast.
I hastily threw together a picture to help illustrate the long version.
The long version:
I kept your original post in mind to get rid of the "U" inside the aquarium (that is why there are two overflows). The same method can also be applied externally as well.
The light blue at 90% shading is your siphon portion. It is only a pathway. Keep in mind it is under VACUUM so if air is able to reach it on EITHER end the air will displace water and eventually relieve the vacuum of the siphon. If you understand vacuum and siphons, then there is nothing else to say other than to keep in mind it is just a hydraulic pressure equalizer. Add pressure to one side and it relieves it by allowing it to push water in the siphon along the pressure gradient.
The dark blue at 40% shading is the aquarium water. When the level is above the overflow mouth it drains into the intake and forms what I call the water table (Dark blue at 90% shading). The water level inside of the intake will be determined by whichever point is lower (intake vs discharge). The equilibrium point is the red line. At that level the water level will balance out. If water is added to the aquarium, it overflows into the intake. The water table rises on the intake side creating a pressure gradient which pushes water out of the discharge end. If your intake is higher than the discharge (pictured) then the water level in the aquarium will be set at that level because it will just be a series of waterfalls. If the intake is lower than the discharge, the water level will be elevated above the intake and rest at the discharge level.
The water table must always be higher than the siphon intake otherwise the siphon vents and breaks.
Now to throw in another hydraulic rule.
Water pressure is ONLY related to its height and density, the density is consistent so we can forget about accounting for it. The larger the difference in height between intake and discharge water levels, the more effectively it is pushed from one side to the other (more pressure). This will be true whether you have long or short tubes because the pressure is still the same. The difference comes from the length of tubes containing more water which generate more inertia (resistance to change in velocity). As speed increases, resistance within the pipes increase also. By having longer tubes, you can reach higher flow rates because internal friction becomes negligible compared to the siphon's ability to maintain equilibrium. Your net pressure does not change, the loss in pressure due to friction gets reduced once the increased mass gets moving.
Now, here is the response to your latest post. Sometimes 2+2 doesn't equal 4.
If the two overflows are completely isolated from each other on the discharge side, then they move water in the 2+2=4 fashion. If the two come together in a "T" then your final discharge rate is the capabilities of that single pipe they T into as well as the loss due to two opposite flows crashing into each other and causing turbulence. I learned that lesson the hard way when a temporary overflow/heat exchanger first backed up because of the loss in flow and then shook itself apart from the turbulence. If the two discharges come together on the same level as the water table then there isn't as much loss from the collision because of a lower water velocity. But there will always be loss unless you can have them "Y" into a larger pipe. There are some pipe fittings designed for home use that do this that are usually in a "y" fashion. This does help tremendously if you want to maintain similar pipe sizes but the final discharge will still need to be larger than the two overflow discharges if you don't want to take a hit on the discharge rate.
The worst case scenario from overflows is not draining the tank. It is not having a high enough discharge rate to keep the tank from overflowing.
[insert horror stories of clogged and vented siphons]
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