"venturi Effect"

[nellie_nospu]

Hello
I recently purchased a new filter. It came with a spout "attachment" which apparently uses the Venturi effect to add extra oxygen to the water. It just looks like plastic tubing to me! It runs from where the water is expelled up to above the water surface. However the water level in the tube is below the surface. I thought it maybe pulled air in and added bubbles to the expelled water - but it doesn't. Is it supposed to? Otherwise the surface area within the tube is too small for any useful or significant gas exchange.

Anyone??

 

[nmonks]

What a venturi does is mix air with the water at high pressure. The trade-off is that pressurising the water means the filter pump has to work harder, so you end up with reduced water flow (i.e., turnover = less biological filtration). If you look at the packaging for your filter, it will likely explain this. Certainly did when I last bought a venturi device around about 1990!

Anyway, do they have a big effect? Probably not. I'm not aware on any studies either way, but I'm happier using plain old spray bars and spouts to cause splashing at the surface of the tank. Moreover, I believe that turnover trumps everything else, since the critical issue is getting deoxygenated water from the bottom of the tank to the top. Once it's there, gaseous exchange should happen readily enough.

On the other hand, some fish seem to like swimming into venturi outlets, so if you have one of them anyway, certainly try it out and see if it works for you.

Cheers, Neale

 

[Dave Spencer]

The venturi on your filter utilises the principle that when you increase the velocity of a fluid, you decrease the pressure. Due to the water`s velocity at the air injection point as it passes, there will be a pressure drop that draws the air in to the stream of water. I have seen the principle used on internal CO2 reactors, but like Neale says, if it is intended as a means of oxygenating water, good circulation and surface disturbance should do the job for you.

It`s years since I did any studying of Bernoulli`s theorem, so I hope I haven`t got it wrapped round my neck.

Dave.

 

[Bignose]

No Dave, you have Bernoulli's theorem right. A Venturi is a specially designed pipe fitting to gradually contract and then gradually expand to minimize energy losses in the fluid. Sudden contractions and expansions have very high friction and hence high energy losses; and if you have high energy losses, you'd need a larger pump to keep the fluid flow going.

I just can't seem to wrap my head around the description the OP left though. Does someone have a picture? I can't seem to figure out where the tube goes, or comes from, or anything.

 

[nellie_nospu]

Hi,
It sounds like it's not doing what it should be doing! All it said in packaging was a sentence along the lines of "There is also supplied a Venturi attachment to increase oxygen levels."
No diagram, no nothing. I just plugged together what fitted and looked vaguely like the box photograph

I will try to get a picture, or failing that do a photoshop!

A related question then - if surface disturbance is best, what is the best position in a tall skinny tank for the filter? At the minute it's towards the bottom as I figured that's where the particulate waste would start settling / be concentrated.
In my long thin coldwater tank my filter is set to disturb the surface so not a problem.


ETA: http/www.aquaone.co.uk/elements/filters/maxi.jpg
These filters are the same, on the far left is one attachment (normal), then on the far right is the Venturi attachment. The little "spike" sticking up out of the spout is the tube that the plastic tube fits on to. This plastic tubing then runs up so it comes above the surface of the water. Make sense?

 

[Matty P]

Doe spray bars decrease the turnover, or is it just the same with one?