Backyard Aquaponics
A place to discuss aquaponics

Yep, the 9.81 is m/s2 and the d is in m.

@Bunson not wanting to be picky but that is not a venturi. Venturi's require a much greater pressure differential. Certainly gets the air into the tank though.

There is no set pressure to classify a venturi. A venture can be ANY increase in velocity.

I never said my system was a Venturi, but I did have a Venturi installed by reducing 40mm to 25mm then expanding back to 40mm which provided a bit more air than the drain as it is now, but the reduced pressure reduced the "push" on the water so the swirl in the Ft was slightly less than it is with the current configuration.

But then again, what constitutes a Venturi? The Bernoulli effect, or any instance where a gas or liquid is injected into a flow? We have venturis as bailers in boats without accelerating the passing water flow, and many more examples.

What you have there is water being entrained like you get in a bath when you pull the plug. The venturi effect occurs when you have a decrease in velocity most often caused in plumbing by a sudden increase in the diameter of the pipe. It obviously does what you want it to but it is the result of a different effect.

When you add a venturi to a drain you add a restriction which causes an increase in velocity but the venturi effect does not come into play until you get past the restriction and the pipe diameter increases causing a pressure drop which if there is another pipe connected in the right stop creates suction. Theoretically the pressure gain is balanced by the pressure drop but due to friction and other factors you get significant losses which means that the differential pressure you need to get significant suction is significant, ie increased pumping costs. Now just because they are increased doesn't mean that they are expensive. It depends on the application and design. Generally speaking though I havn't seen an example were using a venturi for aeration was the most efficient method available. Generally speaking you are better off getting an efficient water pump to pump water and an efficient air pump to pump air.

What you have there is water being entrained like you get in a bath when you pull the plug. The venturi effect occurs when you have a decrease in velocity most often caused in plumbing by a sudden increase in the diameter of the pipe. It obviously does what you want it to but it is the result of a different effect.

When you add a venturi to a drain you add a restriction which causes an increase in velocity but the venturi effect does not come into play until you get past the restriction and the pipe diameter increases causing a pressure drop which if there is another pipe connected in the right stop creates suction. Theoretically the pressure gain is balanced by the pressure drop but due to friction and other factors you get significant losses which means that the differential pressure you need to get significant suction is significant, ie increased pumping costs. Now just because they are increased doesn't mean that they are expensive. It depends on the application and design. Generally speaking though I havn't seen an example were using a venturi for aeration was the most efficient method available. Generally speaking you are better off getting an efficient water pump to pump water and an efficient air pump to pump air.

Are you guys talking about a venturi on an outlet pipe?

I thought you were talking about a pump line, but how would the air get in there?

We use venturi pump on the helicopters. A venturi causes an increase in velocity, for example all fuel systems use a venturi. Technically what I have is a venturi, it has a larger chamber followed by a smaller chamber then to a larger chamber. It really is no different than the venturi in a carberator. In this case the pump impeller is acting exactly like the pistons in the engine pulling the air through the carb increasing the speed of the air through the smaller chamber (the tubing). So you are telling me that this pump with aeration does not fit the definition below? If so please explain.

constriction in tube: a constriction in a tube designed to cause a pressure drop when a liquid or gas flows through it

No it it does. It is just the bit that you are calling the venturi is the pipe that is delivering fuel to the venturi.

This diagram is not very good but it was the best I could find quickly:

http://en.wikipedia.org/wiki/Eductor-je ... escription

See how the motive fluid is coming through a nozzle. This is done to increase the velocity so that when the motive fluid leaves the nozzle there is a bigger decrease in velocity which creates an area of low pressure around/behind the nozzle entrance. The length of pipe that has motive fluid travelling at high velocity needs to be kept to a minimum because the longer it is the more you lose energy to friction because friction losses is proportional to the square of the velocity. That is why commercial venturis use nozzles. Quick, clean, SHORT, contraction. You could make a venturi where the motive fluid was delivered through a narrow pipe 10m long if you so wish but the pump required to make it work would be much, much larger than if you had a large diameter pipe that contracted to a smaller one for the venturi.

I trialled a couple of other ideas than my current (simple and works) configuration.



V1 (left) works OK if the discharge end of the drain is above water level (or very shallow), but as I run my drain to the bottom of the FT, the added backpressure means the extra air draw is almost nil; in fact water dribbles out of the air inlet.

V2 (right) works just as well as V1 when the discharge is above water level, but works much better than V1 when the discharge is at the bottom of the FT. However, as I have previously mentioned, when using the 40mm pipe the "push" for the swirling action action is reduced compared to the 25mm configuration I am using, although it does add just a little bit more air than my current configuration and V1.

I am yet to actually build and test this design (I have been poorly of late) but I am going to try adding a 'T' to the bottom of the discharge at the bottom of the FT, plugging on side and pushing through a small capillary pipe (I have some 9mm lying around somewhere, but I know I have some 6mm inner diameter, larger the better though). In order to reduce the friction with the air being drawn into the venturi from the surface and hence improve the volume of air being introduced, instead of running small piping to the surface, I will envelope the air intake inside a 25mm riser. When the pump is off, the whole drain will flood, but when the water flow starts, the venturi effect should quickly empty the water and begin sucking air which will be inducted/injected into the water flow.



I'll let you know how it goes...


Scott

Bunson, so you would be relying on the flow of the water to create suction on the 6mm tube, right? If flow stopped with water in the pipe, then the water would back up the 6mm tube and into the second stand pipe. I'm also assuming the aid stand pipe on the left would be a little higher so it would never fill with water when water was flowing down the right pipe.

But this also isn't a venturi because it doesn't have a pipe size change / nozzle, correct?

If I'm correct, would something like this be easier? Simply glue / silicone a piece of aquarium tubing to the inside of the stand pipe. That or feed it in through the top of the siphon?

Yup, I am prepared to accept the flooding because it will be quickly sucked out when the pump starts again. The venturi should be as effective on the water as it should be on air (I know the properties are different, but you get what I mean...) The piping on the left is, for want of a better word, a plenium. I don't think that I have all the maths for this right just yet, but I figured it would take about 18 seconds to empty the plenium, but that's at full flow and the flow starts more gently through the GB drain. In any case, I don't consider the time taken to be significant.


The length only needs to be such that the top of the piping is above the surface of the water at all times.


Yes and no, it depends. What defines a venturi? Some people say for this to be called a venturi there has to be a reduction in the diameter of the pipe which increases the velocity through that section creating a low pressure zone which means gas/liquid can be drawn into the flow. Some people say any change in velocity which creates a pressure drop to induct gas/liquid into a flow is considered a venturi. I don't want to argue the semantics of it -- it's not important and I don't really care. It working as designed is the only important part I am concerned with.


The problem with your design is the friction on the air being drawn in through such a long narrow pipe, and vinyl tubing is quite "sticky" ie has a high friction coefficient, so will impede the flow further. My idea is to minimise the friction on the air being drawn in , to maximise the volume of air being drawn in. In my design, the length of the narrows is minimal for this purpose.

I can theorise on this all day; one test will tell me if I'm right or wrong.

So if this works on a flood drain, then it should would in a continuous flow / continuous flood GB as well, correct?



I'm not trying to argue, just understand. I had not heard of them in aquaponics till this thread and wanted to understand the physics and terminology better.



I think you may be giving friction too much credit in this case. With the small distance and low air flow, friction should be pretty low as well. But I guess every little bit helps.


[/quote]

Post it please. I'll keep my eyes out for it, but I would appreciate it if you would PM me if it works or not. I'm interested to see how the Bunson Venturi Snorkle turns out.

Yup.


Google "venturi" or "Bernoulli's principle"


The margins are pretty low already, so as you said, every little bit helps.

The only reason I am doing this is because I have the time whilst I recuperating; otherwise, I am happy with the configuration I have now as it is simple and effective. Do I really think it's going to be THAT much of an improvement over the current configuration? Not really. But, it is great time waster to think, engineer, model and test whilst I can do little else.


I wouldn't go adding my name to it...

If you read my system thread, you'd see that I invented aquaponics from scratch whilst hiking in South America. Had I put my name to it, I would have been sorely disappointed that someone had already discovered it long before I had; I just re-invented something which I didn't know existed.

I'm more mobile today, so I'm going to pick up some stuff and give the initial design its maiden test. Will post results when I can.


Scott

... Indeed Bunson.... a lot of people over the last couple of years have popped up claiming to have invented aquaponics...

And most of them have (apparantly)... been "doing it"... since Noah was a boy.... you wouldn't have even been close mate....

But the main differences between me and "them": I made my discoveries from first principles and did the modelling and research first, then discovered AP already existed! Some less scrupulous people out there did it the other way around.

Venturi Snorkle
FAIL.

Here's what didn't work...
ID/OD(mm)
5/7 (acrylic)
6/9 (plastic)
13/15.5 (polypipe)

Why? I made a fundamental error in my maths where I forgot to include the pressure the drain water was working against the the bottom of the FT. In my configuration, although the "head" from the top of the GB standpipe to the bottom of the FT is about 1m, the height of the top of the GB standpipe to the top of the water is only half of that. A rookie mistake, almost too embarrassing to admit.

All three models work to some extent to induct air in the water flow when the discharge is above the water (hard to measure which was best) but all failed when the discharge was submerged deeper than just a few centimetres.

By adjusting the length of the poly injector pipe, I did manage to get the system in a sort of balance where the water didn't flood back into the plenium part when the discharge was at depth, but no more air was inducted into the stream, so for the extra complexity and cost there is no advantage or gain.

... but it did waste some time, and certainly beats sitting around doing nothing!


Scott