Skimming Principles

[tinygiants]

Where did you find the 13%? The info I found quoted 20%.

I am still in design phase, so it is all just theory. I have not yet bought the air pump.
The PVC is just around the block, I may go get it this weekend.

 

[barryhc]

Where did you find the 13%? The info I found quoted 20%.

I am still in design phase, so it is all just theory. I have not yet bought the air pump.
The PVC is just around the block, I may go get it this weekend.

I think you are more up on this than I am "tinygiants", but again this is the ratio of oxygen to water in the "dwell chamber".

I think that I may have actually found this on a link within the links that you have provided here, I am not sure.

I wil be "off-topic" here for several days, but I promise you that if I find the link again, or anything similar, I will post it here right away

Nice thread, BTW, I'm mostly just following, you are "The Boss" here. > barryhc

 

[ChemE]

It seems that most DIY'ers here at RC are familiar with the fact that some proteins need up to 120 seconds to become trapped at the bubble wall but I think the everyone is applying it incorrectly. We do calculations for flowrates in order to ensure that water spends at least 120 seconds in the skimmer but I think that what we really want is for bubbles to spend 120 seconds in the skimmer.

Think about it, the protein is in a differential volume of water which is moving very slowly down the reaction chamber. All the while it is being bombarded with very very fast moving bubbles. This doesn't result in 120 seconds of contact time with a bubble wall.

The alternate scenario is the same protein in the same differential water volume travels to the bottom of the skimmer and comes in contact with a bubble just as it is formed. Chemical engineers use the "no slip" boundary condition which theory predicts and experiments validiate. This no slip boundary condition states that flow at a surface is stagnant. Therefore the water right at the bubble wall remains at the bubble wall as long as that bubble exists. Once the bubble pops the water at its wall is released back into the bulk fluid and is mobile again. Don't believe the no slip boundary condition? Put talculm powder on your hood before you leave for work Monday morning and drive 1000 mph to work; it will still be there when you arive very very early. Anyway, if the bubble takes 120 seconds to travel up the skimmer the protein's weakly hydrophobic section has a chance to move into the bubble thus capturing it.

Now, I of course realize that 120 second bubble rise time calls for a 100' tall skimmer but perhaps a very very low turbulence recirculation design in which 90% of the bubbles gently recirculate in a wide diameter loop (minimal turbulence) is the ticket. The only real issue I see here is keeping a high enough downward flowrate in the recirculation portion that the bubbles are entrained by the water and carried down and around rather than just building up or running back up towards the water surface. If it takes a bubble 10 seconds to make the recirculation circuit and there is a 90% chance of taking the circuit then 0.9^12 bubbles will be in intimate contact with the water at their surface for 120 seconds. See where I'm going with this?

OK, since this is contrary to conventional wisdom let the dissent begin.

 

[tinygiants]

Interesting points there ChemE. I had not considered the info in that way. If the 120 seconds is a valid number, and the bombardment rate is a valid ratio, the water dwell would have to be 20 minutes.

Water dwell / air dwell = bombardment ratio (goal of 10 according to Aquatic Systems Engineering by Peter Escobal)

water dwell / 120 seconds = 10
Water dwell = 1200 seconds = 20 minutes

With a 2 gallon feed rate my skimmer would have to be 40 gallons big to have that water dwell. In a 12 inch body, that is over 80 inches tall. This of course does not guarantee the air bubble dwell.

There are so many factors when trying to balance everything.

Dale

 

[tinygiants]

I think you are more up on this than I am "tinygiants", but again this is the ratio of oxygen to water in the "dwell chamber".

I think that I may have actually found this on a link within the links that you have provided here, I am not sure.

I wil be "off-topic" here for several days, but I promise you that if I find the link again, or anything similar, I will post it here right away

Nice thread, BTW, I'm mostly just following, you are "The Boss" here. > barryhc

"According to Escobal, the upper limit of the amount of air able to be inside the skimmer at any one time is 13% of the water volume inside the skimmer."

I found the above quote at this website. Looks like you were right. My original info was wrong.

 

[Whaledriver]

It wont be the time one individual bubble takes. It is the average life of all the bubbles. Just like its the average life of the water flowing through the skimmer. This leads to a hard to answer question. What is the life span of the foam in the skimmer?

In the end we are left with a recirculating skimmer that gives the water the desired time in the skimmer. All we can do is add air to the point of saturation of say 13%. The volume of the skimmer tube is dictated by these two factors. Remember not to count the area of the skimmer that doesnt have foam while operating.

The only other question is how tall and how wide of a reaction chamber area. There is a limit to how high foam can get before it starts to crush itself. ????????? I would guess 3-4 feet BUT would like some help on that. This limit leads you to the very wide European skimmer types with tubes that are up to 24 inches. A wide tube also lets you have a capable skimmer that will still fit under your stand.

Just tossing out a few ideas. hope it helps

 

[CaptiveReef]

Skimming principles

Skimming principles

The best way to get the most dwell time is by the distance principal. The travel of water through the micro bubbles in 4-5 ft
gives better contact time. Also with the skimmer design shown above, the counter current also adds to the contact time by creating a rolling effect of bubbles and water.
Also the diameter of a skimmer is important, if the skimmer is too wide there is a loss of contact time, due to the bubbles are spread out.
A water flow that is kept tight in say a 4-10 inch wide tube will force more water into the mix of bubbles.
A taller skimmer will also allow for a faster flow rate through the skimmer. The shorter the skimmer the slower the water flow.
Whaledriver when you mention the crush principle, that is the flow rate that causes this, you will lose the skimmate production when this happens. Just slowing down the water flow into the skimmer will reverse that.
With my skimmer design, if I go to 800 GPH flow to the skimmer I lose all skimmate production. As soon as I dropped it to 400 GPH it started to foam right away.

CaptiveReef

 

[Puffers]

Also the diameter of a skimmer is important, if the skimmer is too wide there is a loss of contact time, due to the bubbles are spread out.

Maybe I'm misunderstanding something, are we saying that if we go to a larger diameter tube the contact time of the bubbles is less? Is the contact time we are talking about water contact or bubble to bubble contact? Couldn't we in a larger diameter add more bubbles to offset this effect and thus reduce the height of the skimmer? (Keeping the same GPH in mind).

 

[barryhc]

Maybe I'm misunderstanding something, are we saying that if we go to a larger diameter tube the contact time of the bubbles is less? Is the contact time we are talking about water contact or bubble to bubble contact? Couldn't we in a larger diameter add more bubbles to offset this effect and thus reduce the height of the skimmer? (Keeping the same GPH in mind).

It is "water to bubble" contact time.

Puffers, I believe that CaptiveReef may be eluding to the fact that centrifugal force will tend to cause a gradation of "light to heavy" from "center to periphery" in any swirling gas or liquid. Larger dias. will definitely increase "contact time", and obviously "shorten" the unit. Gradation of "oxygen saturation" can be controlled in this respect, by introducing the air near the center.

The "collection point" or final "path" of this air-water mixture, would also have a great deal of impact on "dwell time", and skimmer "performance".

> barryhc

 

[Puffers]

Hmmm so much to learn yet no money to experiment with the knowledge gained.

I guess I was thinking along the lines of an air driven skimmer where the water column is just vertical movement.

I think I understand what you are getting at barryhc. Would it be possible to spin the water fast enough to have all the waste be removed (or more concentrated on the outside diameter) without any air? Maybe attach a wet/dry vac to our skimmers to suck just the super concentrated "waste/water".

Or just use that Tunze skimmer....

 

[barryhc]

Hmmm so much to learn yet no money to experiment with the knowledge gained.

I guess I was thinking along the lines of an air driven skimmer where the water column is just vertical movement.

There is nothing inherently ( or otherwise ) wrong with that. DIY is the "great financial equalizer"!

I think I understand what you are getting at barryhc. Would it be possible to spin the water fast enough to have all the waste be removed (or more concentrated on the outside diameter) without any air? Maybe attach a wet/dry vac to our skimmers to suck just the super concentrated "waste/water".

There will already be less "air" at the outside, and a "vertical annulus" could be incorporated for "large particulate collection".I don't think that would take any more "speed" than what is normally in any venturi type skimmer. Just "catch" the "particulate-laden" water at the "top-outside" edge of the skimmer body, and "collect" it. The "skimmate at this point would be very "wet", and might need to be turned "on-off" at some interval that would keep up, or especially "just AFTER feeding time" ( etc. ).

What do you think?

> barryhc

 

[Puffers]

Could this set-up miss a lot of particles? Not everyone of the particles will be on the outside edge. This is why I was thinking of increasing the speed of the rotation to help force all of the particles towards the edge.

Perhaps an improvement could be to take the water from the center of the water column and leave the rest of the water spinning in the skimmer body. Thus giving more time for the system to catch more particles and wet skimmate. Remember in this set-up there is no air in the skimmer, perhaps this is where I am getting off track as you.

Wish I know how to draw idea this on RC...

 

[Whaledriver]

The goal of a protein skimmer is to remove protein from the water.
To do this bubbles are created that attract the protein molicules to their surface.
To make a more efficient skimmer we need to a long contact time with the bubbles surface. Some proteins need up to 2 minutes to bind with the bubble.

So we end up with a situation where the goal is to creat a gentle foam that can slowly rise to the collection cup fo removal.

Foam will be compressed by the weight above it.

Look up a Bubble King skimmer and see one of the best reviewed skimmers look like.

 

[Puffers]

Could the goal be expanded to include removing large particles and proteins? I still will need air in the skimmer for it to work properly in my previous post.

By increasing the diameter of the skimmer does increase contact time.

So we end up with a situation where the goal is to creat a gentle foam that can slowly rise to the collection cup fo removal.

Foam will be compressed by the weight above it.

When I think of a wave crashing on a shore it doesn't seem gentle, but it does have a long contact time to achieve all that protein contact.

I'll check out that skimmer.

 

[barryhc]

Could the goal be expanded to include removing large particles and proteins? I still will need air in the skimmer for it to work properly in my previous post.

By increasing the diameter of the skimmer does increase contact time.

I think what you are looking for, could be done, but before "we" decide that it's important, I think we should define the objective.

If I understand your "position", what you're wanting, is to get large particles out of the "water column" right away. Please tell me if this is your concern.

One thing I think would be helpful, would be a tapered section at the top of the "dwell chamber", or "main skimmer body". This would tend to direct the large particles "up" into the "lifting neck" for disposal, instead of them "languishing" in the "upper corner". This may not be an easy construction method, but let's "dwell" on the "objective", before we let construction difficulties interfere.

Another idea that is gaining popularity in europe, and on particularly large systems, is "wet neck" technology. The aforementioned "taper" would also improve "wet neck" functionality.

I can "expound" if you are interested, but let's be sure that your "objective" is going to be met, first.

> barryhc :beachbum:

 

[Puffers]

Yes the main objective was/is to remove larger particles, the thinking of this is to create a skimmer that is perhaps better suited to specific environments, an example could be BB.

Currently in my HOB skimmer the large particles will/can get collected inside the skimmer cup, but they always don't end up in the cup.

Part of this is because the bubbles needed to "raise" the particle burst/break because of the weight/size of the particle. In addition the needle wheel will chop the large particles into micro particles which will coat the insides of the skimmer, making it a bit harder to get good foam production.


:bum:

 

[barryhc]

Yes the main objective was/is to remove larger particles, the thinking of this is to create a skimmer that is perhaps better suited to specific environments, an example could be BB.

Currently in my HOB skimmer the large particles will/can get collected inside the skimmer cup, but they always don't end up in the cup.

Part of this is because the bubbles needed to "raise" the particle burst/break because of the weight/size of the particle. In addition the needle wheel will chop the large particles into micro particles which will coat the insides of the skimmer, making it a bit harder to get good foam production.

:bum:

Ok, well I may be sticking my neck out a bit here, but here goes.

I have done a tremendous amount of investigation on skimmers, very recently, and air driven skimmers seem to be the big winners here. Airstone technology has progressed by many factors, in recent years, and along with good incoming air filtration, maintanence of these skimmers has become generally less problematic than those incorporating needle wheels, becketts, etc.

Larger particles can be moved up in the collection cup, with any type of skimmer, if the collection tube is shortened, and/or if wetter skimming is utilized.
Wetter skimming has gained much popularity recently, and for good reason, especially for removing larger undissolved particles.

"Wet-neck" technology is inherently wetter than standard skimmers, but this becoming more of an asset recently, than a defecit.

Well, I suupose I have stuck my "neck" far enough out here already, so tell me what you think.

> barryhc :beachbum:

 

[tinygiants]

From my research, you arre right on. Air driven skimmers are meeting more of the basic "Rules" I have found. The biggest draw back to efficient design is height. Air driven skimmers get one shot at dwell time, while reciculating designs get dwell time in shorter bodies. For me, my sump is in the basement.

Dale

 

[ChemE]

Has anyone ever used a Tunze to recirc an airstone skimmer? This would only add 11 watts to the equation but could make for some incredible dwell times. Let me hear your thoughts...

 

[barryhc]

Well Dale, that dwell time might lead us "back" to a larger dia. to accomodate "dwell time requirements. Various "wild a$$ options" come to mind here, for dwell time, in conjunction with "large dia.-short" skimmers.

There is no reason why we can't incorporate recirculation, in an air driven skimmer, but the design might be very far away from what is considered "normal".

Give me a "say what?" on that one. > barryhc :beachbum: