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Wednesday, September 7, 2011

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 didn't 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.


  1. Well, it worked. At first the water started flowing over the standpipe, seemed to be suctioning, but it broke at the height of the standpipe. Fix: opened the spaces at the bottom of the middle pipe (with the cap) allow more water to flow up the inside of that pipe. That fixed it! Tested with roughly 180 gallons of water [empty growbed-no rocks], sucked it all down in about 20 minutes with heavy flow. Will need to increase the overflow piping into the sump tank to handle the worse case of water,if both growbeds flow into the fish tank at once. the net height of standpipe is 10.5 inches in a 12 inch growbed. Very happy! Thanks Meg, for the video.

  2. Congratulations - it's always wonderful when things work!

    Since I posted this, I've seen pictures of older systems (all in Australia, I think) with a 45 degree elbow. But I do think I'm the first one to explain *why* it works.

    I'll be interested to see how large of a grow bed can be effectively drained with 3/4" (19mm) pipe. A lot of folks swear by 1" or 1.5" pipe, but I figure if 3/4" works, it's better to go with the less expensive option. Besides which, I love the fact that it takes up less space in the grow bed.