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dg84s Offline OP
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My first post... so be kind.
I am trying to determine what the head pressure would be at the pump in the example in the attached image.
A brief description: Pump will be placed in workshop 500 ft from pond, with 1" dia buried poly pipe to manifold at edge of pond where dia will be reduced to 2 separate 100 ft 5/8" weighted air lines running to 2 Vertex XL2 diffusers 10 ft deep.
Will the pressure be 5+1+.4=6.4 psi? If so a Gast 0523 pump should give me ~3.8 CFM to drive the two Vertex diffusers. Or will the pressure will be 5+5+1+1+.4=12.4 psi? If so, I will need to choose a different pump.

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Adding two diffusers would not double the pressure. It simply divides the volume. The pump is looking at the same depth pressure no matter one or ten diffusers. Also having two hoses reduces the overall line resistance compared to a single5/8.

Snrub

Last edited by snrub; 10/20/13 08:33 PM.

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Forget about any pressure calcs for the line that is above the water level. Add .5 psi for every foot the diffuser is under the water surface.

i.e. If the diffusers are 10' below the water surface, you would see 5 psi on a gauge.

Like was said in the previous post, it doesn't matter the number of diffusers.

The diffusers should be placed so the membrane is right side up, and not covered by anything. I ran into a situation on Friday. I pulled some diffusers from a pond, and they were placed upside down in a 5 gallon bucket, and covered with gravel to weight them down. The lifting rate of the diffusers had to be affected because the bubbles were condensing into larger bubbles as it went thru the gravel.


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Esshup, just a quick question. I have been reading about diffusers and in fact just placed a double Matala 9" at about 9' a week ago as an initial experiment in evaluating if I want to go the full route of aeration of my entire pond. So I have been reading up on pump performance curves at various depths, line loss and size, etc. etc. But all my learning up until I actually put the diffuser in the pond and hooked it to an inexpensive compressor has been book learning where you no doubt have lots of practical experience (my temp setup seems to be working quite well and creating lots of water flow).

From my reading, it seems each foot of fresh water adds .43 pounds pressure. In other words an atmosphere of fresh water is about 34'. The Matala specks on my diffuser says it only adds about 6" water depth. In other words the pressure it takes to move the membrane is equivalent to .43/2 or .22 or about a quarter pound of air pressure. The line losses are kind of fuzzy to me. I know by hydraulics understanding that in hydraulic hoses hose size is very important because friction losses get high as velocity of fluid increases and the smaller the hose, the faster the velocity has to be to move a given volume of fluid (in fluid it takes four times the pressure to double the flow volume for a given diameter orifice). I am completely unfamiliar with friction losses in air but I know they exist. Don't know if they would be similar to fluid of not.

For those considering some of these issues I find illustrations helpful. Imagine putting a soda straw a foot long in your mouth and trying to breathe through it. You have to suck and blow much harder (takes more pressure for the single straw than the double) to breathe through a single straw than if you had two straws in your mouth. It would be easier yet with a foot long 5/8 garden hose and no problem at all breathing through a foot long piece of 2" PVC pipe. This is an illustration that I use in my own mind to help understand the importance of line size. If a person is at rest, breathing through a single soda straw is doable (low cfm requirements). If working hard it would take a 2" pipe to breath through to not restrict your breathing (high cfm requirements). This is all pretty easy to see.

Now the trick is transferring this understanding to using the correct line size for the pressure and flow in a aeration system. LOL Generally it seems the flow rates are pretty low in aeration systems, so is line size not too important? Does length of line come into play a lot?

I lied. It was not a simple or quick question question. Ok, finally to the main question. You use .5 for your pressure rather than .43. I am assuming that is just a ballpark figure that is shorthand for determining what the additive effects of the depth, line resistance and cracking pressure requirement of the diffuser? If this correct?
Sorry for the longwindedness.

snrub


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Yep, .5 is a ballpark figure. Better to overestimate the pressure than underestimate the pressure. More wiggle room so to speak.

Ballpark: 5/8" I.D. up to 300' run, 3/4" 300'-500', then go to 1".

What is the CFM rating on the double Matala 9"? That will give you an idea of volume.

Do you have a pressure gauge on the outlet side of your compressor? If not, plumb one in. It is a great diagnostic too., and a way to monitor the diffuser performance. I'd get ont that reads 0-15 or 0-25/30.

For a learning experience, take the air hose that you are using, disconnect it from the diffuser and float it on the surface. Read the gauge. Place it in the bottom of the pond. Read the gauge. Connect it up to the diffuser, take another reading. Verify that their specs are correct. If so, you should only see 1/4 pound change in the pressure gauge. I'm interested in what readings you see on the gauge.

Your straw anology is good, but the volume of air has to be considered. Air could be considered a fluid, but just not as dense.


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Thanks for your help.
In my mind I knew that the cfm would be halved and pressure unaffected by the second similar diffuser at the same depth and air line length. I just needed reassurance. And...yes I know the membrane points to the surface.
I'm off to buy my pump, air line and diffusers. Thanks!

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The Matala dual 9" diffuser recommends 2.8 to 8.4 cfm (1.4-4.2 per 9" diffuser head. If I figured right on the pump output, at the 8.5' depth I should have about 3cfm which is on the low end of the range.

I'm actually not using this pump but a Pondmaster 100. They do not provide a performance graph but the specs they do supply seem to match up pretty well with the Hakko.

I could have went with the single diffuser but thought the double would provide smaller bubbles and move more water for the air flow. I can change it to a single easily if needed but it seems to be doing fine. On a calm day I can pedal up to the diffuser on our Paddle King paddle pontoon boat and the current quickly pushes the boat away. Seems to be moving quite a bit of water.

Eventually will probably want three diffusers in the 3.5 acre kidney shaped pond with a bigger pump (or multiple small ones) but I wanted to get something this fall to try out to see if it was something I wanted to pursue further. I ran 3/4 100' then 50' of 5/8 sinking.

Pressure gage is good idea. 15 psi on the way here. When I get it will do a little playing as I have time and see what the results are.

This pond started out as a casual remark to wife that a small pond behind the house we could look out over from our kitchen and patio would be nice. What did she think?. Her remark was something on the order of "when are you going to start, how big will it be and when will it be finished". I thought about 3/4 to 1 acre would be nice. "Not near big enough. Bigger. Much bigger". She is always right. LOL

What started out to be a little farm pond has taken on a life of its own. We are enjoying it though, so I guess that is what matters.

snrub


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I did check it with a 15 psi pressure gage last fall (right before I shut the system down for the winter) and now it has been so long I forget what it was, but it was within a couple tenths of what our calculations said it should be. I figured that was within the range of accuracy of what the gage probably was and/or the depth measurement of the water from the top of the diffuser.

Last edited by snrub; 01/21/14 11:01 AM.

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For those that are interested in how to figure psi of a fluid column I added an explanation we use here at work.

1 cubic foot contains 7.48 U.S. gallons.

A fluid weighing 1 ppg would weigh 7.48 pounds per cubic foot.

The pressure exerted by one foot height of fluid over the area of the base would be:

7.48 (lb\f) \ 144 }(in^2)} = 0.0519 psi (rounded to 0.052).

Therefore, a one foot high column of 1 ppg fluid would exert 0.052 psi on its base.

Fresh water weighs 8.3454 pound per gallon.

0.052 is the pressure gradient, psi/ft, of 1 lbf/gallon fluid.

So with my diffusers at 8' then the hydrastatic pressure on my 9" disc is 8' x .052 x 8.3454 ppg (fresh water) = 3.47 psi.


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