Skimming Principles

[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]

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]

Originally posted by Puffers
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]

Originally posted by 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.

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]

Originally posted by 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.

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

[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.

[barryhc]

Originally posted by 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.

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

[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

[barryhc]

Originally posted by 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...

Well, Mr. mad "scientist-engineer", welcome to the "fray"!

I don't know why you would particularly pick a "Tunze", but then I don't know that much about them, yet.

Please "expound" on this idea, and we will chew it up, and spit it in whatever direction that seems appropriate, including "straight ahead"!

Thanks for "coming in", > barryhc

[ChemE]

I specifically picked a Tunze because one can get 1600 gph with 11 watts at a low linear velocity with little to no turbulence (think low speed underwater fan). This is ideal given a large diameter recirculation loop (I'm thinking the same diameter as the skimmer body so as not to increase the flow velocity too much). As we all know, turbulence is our enemy when it comes to removing those weakly adherent proteins yet recirculating skimmers always use insane mondo pumps which beat the froth to death; little chance that those proteins remain captured by the bubble wall in the face of so much turbulence.

So what I am proposing is a wide, low-turbulence, slow recirculation loop driven by a very very energy efficient pump (airstone skimmers are low electricity and that is why I like them, want to keep it that way). The flow velocity down the recirculation loop needs to be just greater than the incipient velocity of the air bubbles (the velocity at which they become entrained by the fluid are are thus dragged downward through the recirculation loop). Sorry for all the technical lingo, you know us engineers just love to drop the science.

Chew away...

[ChemE]

Barryhc,

I actually joined the "fray" on the first page on a similar point; contact time.

I'm still unconvinced based on my knowledge of fluid flow and chemistry that what we are shooting for is 120 seconds of water dwell time. A fairly straight forward surface chemistry arguement would indicate that what we really need to strive for is 120 seconds of contact between an air bubble and the bulk fluid.

[barryhc]

Originally posted by ChemE
Barryhc,

I actually joined the "fray" on the first page on a similar point; contact time.

I'm still unconvinced based on my knowledge of fluid flow and chemistry that what we are shooting for is 120 seconds of water dwell time. A fairly straight forward surface chemistry arguement would indicate that what we really need to strive for is 120 seconds of contact between an air bubble and the bulk fluid.

I may be wrong here, and I may very well need to review, but I thought that the original "princiapls" adressed averages of "bubble-dwell, factored to "water dwell", and then "bombardment rate".

Now, without having done this review, yet, "Bombardment rate", may be the point that is being currently covered to some degree by "looking toward" air driven skimmers.

In any case, I follow your point to some degree regarding contact time of an air bubble to the bulk fluid. However, this contact time with the bulk water, will not be adequate if we only pass one ounce of bulk water past the bubbles, per day. Now I'm using the "infinity argument" here, but you get my point.

Actually, the 120 second time, I think is "subjective", and some "dissolved compounds" need more of a "gentle path" in order to attach, than others, and this is getting "quite heavy" to say so, but nonetheless, skimming technology has progressed now , to this point of "finness".

I certianly appreciate your response, and I apologize for not having noticed that you have been involved since "the beginning'.

Keep us "straight" here with your input, It's all good, when we find an answer.

> barryhc

[ChemE]

Agreed, we need to pass the required water flow through the skimmer in order to skim all the water twice a day but at the same time we need to ensure that some bubbles exist for at least 120 seconds in the bulk fluid; this can only be reasonably done with a gentle recirculation loop. It would seem that if both these objectives can be met we would have a design that conforms to all of Escobal's rules.

[tinygiants]

The reason for not recircing an air skimmer is the independent air source. If you need longer dwell time for the water, just slow down the feed rate.

ChemE proposes a recirc not for water dwell, but air dwell. That is a different idea altogether. I am no chemist, but my logic does not follow the difference in 120 seconds of air dwell being different than 120 seconds of water dwell. I followed the thought on it being the airbubble attracting the protien, but that would assume the same molecule of water stays with the same molecule of air for the 120 seconds. If I managed to get a standard design to meet the principals and have 120 seconds of air dwell in counter current design, how would the water molecule stay with the airbubble? Doesn't the bubble time in the foam tower count in that binding time?

Dale

Originally posted by 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

[tinygiants]

Originally posted by 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

Getting back into ChemE's point, meeting all of Escobals principals. If height was not a concern, you could could make the body skinnier and taller till the bubble took the 120 seconds to rise through the 40 gallon volume. (Based on the numbers above) A 8" skimmer would be about 15' tall to get the bombardment ratio right.

Dale

[tinygiants]

Thinking about it more using both barryhc and ChemE ideas, the recirc does seem to be feasible. The water dwell ill take care of itself based on feed rate. However at 2gpm feed in my tank, I am not going to create much counter current to slow down the bubbles. Using a standard recirc of the water to create an increased counter current would lengthen the dwell time. It would also decrease the amount of air needed to maintain the 13% volume if the bubbles stayed in the water for 2 minutes.

How fast would the water have to flow to in a 6' tall body to get 120 seconds of dwell?

[ChemE]

Dale,

To explain the difference between water dwell time and air dwell time reread my post on the first page; it was long winded but I thought thorough. I'll summarize here.

The protein will remain captured by the bulk fluid until it embeds in the bubble wall. This process can take up to 120 seconds. Allow me the use of an analogy that might make things clearer...

I have terribly slow reactions, so much so that it takes me 120 seconds to catch a basketball thrown at me. Throwing millions of basketballs at me very quickly will not help me catch even one. What you need to do is throw the basketballs extremely slowly, so that they take 120 seconds to pass near to me thus giving my slow reflexes time to kick in and catch it. Now, it is impractical/impossible to throw things this slowly since gravity makes them move faster than that so we'll throw me too. If you throw me at the same speed as the ball, then I get my 120 seconds next to one ball and am able to catch it.

It is the same way with bubbles and proteins in the water. It doesn't do us any good to pelt a protein with a cloud of fine bubbles because it doesn't spend enough time next to ONE of them. Sure, it might get pelted for 120 seconds before it exits the skimmer but that is not how embedding works, it needs 120 seconds next to the SAME bubble to embed. If it is in contact with one bubble for 2 seconds and then it moves away and is in contact with a new bubble the embedding process must by definition start all over. 60 such restarts before exiting the skimmer will produce no result (at least not the one I want).

I think bombardment rate is a load of crap unless I completely misunderstand it (possible).

The only thing that makes any sense whatsoever is one bubble being in contact with one protein for long enough that that hydrophobic portion of the protein moves inside the air/water interface and becomes captured by the air bubble. This increases the surface tension of the air bubble and if this surface tension is increased enough, it will not pop until it is in the skimmer collection cup. Then and only then is a protein removed from the system.

[tinygiants]

Originally posted by ChemE
Agreed, we need to pass the required water flow through the skimmer in order to skim all the water twice a day but at the same time we need to ensure that some bubbles exist for at least 120 seconds in the bulk fluid; this can only be reasonably done with a gentle recirculation loop. It would seem that if both these objectives can be met we would have a design that conforms to all of Escobal's rules.

Would a really long loop with the air entrained in the fluid be different than the recirc loop using water flow rate to slow down the air bubble? If the water and air travel down the loop at the same speed (trying to keep the bubble with the same water molecule) wouldn't the airbubble just collect at the top of the pipe?

This is good points being discussed here. I am enjoying this information exchange. Thanks to everyone who is sharing.

Dale