7 The Coanda Discharge

The Coanda Discharge letting out below the water surface
The Achilles heel of the Bell Siphon is getting the silly thing to start.

There is an art to setting the bends in the typical 90 degree elbows of the typical discharge pipe design. If it fails to work in manner A, bend one way. If it fails to work in manner B, bend the other way. Or fuss with the rate of flow into the growbed. Or check the pipes for bio-fouling and rinse with a hose.

No wonder many people just give up on bell siphons and revert to timers!

The Coanda Discharge eliminates any need to fuss with the discharge. The Coanda Effect is that fluid likes to stay adhered to a surface even when that surface bends. It is why planes fly. It is why milk sometimes misses your cup and drips in a strange direction.

I stumbled onto the Coanda Discharge by accident. I planned to put together the kind of right-angle configuration I’d seen in videos of aquaponics systems. But I didnt have a 90 degree elbow, so I used a 45 degree elbow instead. The 45 degree elbow worked great. Because of my fluid dynamics background, I recognized why it worked so well and what it needed to be named.

When water overflows the lip inside a bell siphon, water sheets down the sides of the tube, leaving a column of air. You need to form a plug of water in a location where gravity can pull it out, sucking out the air column and starting the siphon. A 90 degree bends form that “plug” by making the water splash, but when you turn the water 90 degrees from vertical, you’re horizontal and gravity is no longer helping you.

In a Coanda discharge, though, the slug of water is formed by the stable “mounding” of the water as it encounters the pipe inserted into the 45 degree elbow. There’s little to no splashing, and when the plug of water has formed, you still have plenty of gravity to pull out that water plug and the air column behind it.

Here’s a video clip showing the difference between no elbow, the 90 degree elbow, and the 45 degree elbow cases.When the outlet of the Coanda discharge is buried below the water surface in your CHOP fish tank, the only significant noise is the burp when the siphon breaks.

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3 Setting Up the Tanks

Tanks and materials for the 365 Aquaponics System

The first thing you need to have in an aquaponics system is a way to hold water.

In order to achieve system stability and grow an interesting quantity of food and fish, you’ll want to shoot for a water volume of 250 gallons. That’s a lot of water.

There are a lot of options. Concrete ponds, International Bulk Container (IBC) totes, 55-gallon drums, wood structures lined with plastic. For the 365 Aquaponics system, I chose stock tanks.
Here are my reasons for using stock tanks.

They are an existing and proven product. Stock tanks were designed to hold water for cattle, sheep, and other large livestock. They were designed to withstand day to day abuse from such livestock and the elements in which the livestock lives. Because plastic stock tanks are rugged, large capacity, and constructed from food grade plastic, they are often used by restaurants for food storage and preparation. Perhaps most important, they should be locally available.

When you’re buying something this big and having it delivered to your home, you’ll pay a hundred dollars or more just for shipping. If you can get it in stock from a local agriculture store, the shipping to the store has already been covered by the store as part of the cost of doing business.

They require little modification, if any. Grow beds in the 365 Aquaponics System have a single easy to drill hole (1” if using ‘English’ units, 25 mm if using metric). Beyond stacking some cinder blocks and planks, I don’t need to build support structures.

So here’s a video clip showing me preparing the stock tanks for the 365 Aquaponics System.

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