Skimming Principles

[barryhc]

Well, here is "the thing". Many of Escobals reccomendations are based as much on emperical observations as any "scientific" calculation, and some of them particularly so.

I have seen "wet neck" applications that run as high as 2000 gph of water flow, for a 750 gal. tank. That is about 70 turnovers a day VS two. Escobal was not observing "wetneck" systems when he made his observations.

I am not necessarily even promoting "wet-neck" technology, when I say this. Still, you are charting new ground, and further observation will tell "the tale".

> barryhc

[barryhc]

Beautiful job there ChemE, were "passing replies" again, oh well.

I really love the analogy, and I'm going to sleep on it.

Thanks, "tommorow". > barryhc

[tinygiants]

ChemE,
Thanks for explaining the reason for the air bubble dwell being important. Your analagies make sense to the pipefitter in me. I am just having trouble with the concept of the same water molecule with protien staying in contact with the same air bubble. As the bubble moves through the water column isn't the surface of that bubble constantly changing which molecules are on it?

[tinygiants]

barryhc,

I am still in the design phase, but I am trying to do my homework. I am about ready to order my air pump, stones, and uniseals. I have just been trying to make the skimmer as "efficient" as I can. I have a downdraft skimmer that is converted to beckett, but it is weak at best. There is no dwell time for air or water since it is not a recirc design.

[ChemE]

Tinygiants,

In a word: no. The no slip boundary condition used throughout fluid dynamics has been observed time and again (see my talculm powder experiment in my first post to this thread to prove it to yourself). This no slip boundary condition assures us that at the moment an air bubble forms, the water molecules at its surface will remain there until it pops. Now this still allows for proteins to diffuse to and away from this boundary but hopefully they will diffuse to it and then stay there long enough to become captured.

In response to your earlier question about the closed loop, I think that if done properly we could keep a gentle recirculation loop going without getting a build up of air bubbles at the top. But like barryhc says, we need to experiment to know for sure.

By the way, I too am very much enjoying this thread. My skills with DIY are very good but I haven't yet begun practicing on acrylic so I'm going to rely on others to prove or disprove my theories. I am very sure that we can come up with a air driven recirculating skimmer that uses 20 watts yet will overskim a 300 gallon tank. Most of the skimmer designs out there are terrible and have little or no thought behind them. They basically make up for their lack of design finesse through 300-400 watts of power draw.

[Puffers]

I think bombardment rate does come into play. If we only have one bubble in our skimmer that had a contact time with the bulk water for 120 seconds it won't take very much out of the system (unless it's one big bubble). So how many bubbles are too much or too little?

Is there a point where many fine bubbles in a skimmer body decrease the amount of dwell/contact time needed? The thinking here is that because there are so many small fine bubbles there is not much room left for the proteins to go....?

I also think air driven skimmers have come a long way and think there is huge potential in this area of skimming.

[barryhc]

Originally posted by Puffers
I think bombardment rate does come into play. If we only have one bubble in our skimmer that had a contact time with the bulk water for 120 seconds it won't take very much out of the system (unless it's one big bubble). So how many bubbles are too much or too little?

I think that the bombardment rate is the means to an end, and that end is maximum saturation. Maximum saturation is supposed to be 13%. I'll have to check back, to see if this represents water or air. It seems terribly low, for air, when you look at a "milky froth".

Is there a point where many fine bubbles in a skimmer body decrease the amount of dwell/contact time needed? The thinking here is that because there are so many small fine bubbles there is not much room left for the proteins to go....?

I think this may actually be a valid point. Let's think about something here. Depending on the various designs, there are often times, two different "areas" of oxgenated water. Even three, if you like. There is the "foam" in the neck, the "foam" near the top of the skimmer body, and the "bubble water" mixture that is lower in the skimmer body.

Now this second area of "foam", is "leaking" water concentration, and therefore concentrating the skimmate to water ratio.

The third ( or top ) "foam" area in the collection "neck" is obviously 95 to 98% air. The second "foam" area in some skimmers could be as high as 80 to 90% air. This is sitting of course, on top of the first area, which is where I think that most of this "maximum saturation", bombardment rate, dwell times, etc. is going on.

I think that it is this first area, where all of these considerations are taking place, and I wonder if "we" are failing to consider the importance of the "second area", and the "dwell time" that is occuring there.

I don't think it is at all the same as what is occuring in the "lower-first-area". The "foam" in the "collection neck" is obviously getting a lot of dwell time, in fact it self regulates to some degree, and "waits" until it has had enough dwell time ( and water leakage ) to finally "push out the top".

The same kind of thing can be occuring in the "second area", and we may be able to control this to some degree with the design.

It's something to think about. What do you think?

> barryhc

[gbtower]

I don't think you can count any other "area" other than the first when figuring the "dwell time", because that time is what is supposed to be required to have the protein stick to the bubble. At that point, the water should be separated from the air -> which is what is happening in the other 2 "areas" you delineated.

I wonder about the whole 120 sec contact time figure. How was that determined? I would think that a protein would latch to a bubble once it becomes oriented in a favorable way. I don't think there is some other chemical bonding/binding interaction going on. So, is the 120s the average time that these "difficult" proteins take to orient? If so, would a little localized turbulence speed up the time it takes to orient by spinning it around a little quicker?

I'm also not so sure if the no-slip boundary condition would "help" in pushing for more counter current flow, since some proteins may be too large to be contained within the boundary area. Sure, the talc may stick to the windshield, but a significantly smaller percentage of cat hair would.

Moving on - how do we go about trying to remove larger particles with a skimmer? The way I understand it is that we push so much air, that the froth becomes a mech filter, like a simple screen, since there is not enough space between bubbles for the particles to pass through. To do so, we would/should most likely be pushing more than the magic 13% air.

Amd just to throw in something from BB principles, if we want to try to get stuff out before it is broken down, perhaps we should concentrate more on detritus skimmers than protein skimmers (protein skimming would be a secondary effect of the detritus skimmer)? Or is there a better way to get the larger stuff out than oversaturated wet skimming?

Just dumping some mental garbage... You guys put too many ideas in my head.

[barryhc]

Originally posted by gbtower
To do so, we would/should most likely be pushing more than the magic 13% air.

The 13% limit is supposed to be a physical limit that we can't avoid ( or pass ), not a "target".

Amd just to throw in something from BB principles, if we want to try to get stuff out before it is broken down, perhaps we should concentrate more on detritus skimmers than protein skimmers (protein skimming would be a secondary effect of the detritus skimmer)? Or is there a better way to get the larger stuff out than oversaturated wet skimming?

Seriously, some filter floss, that we throw away once a day, shortly after feeding, would perform miracles.

> barryhc

[tinygiants]

The 13% is supposed to be the maximum saturation of the small bubbles we are after before the volume of air causes the bubbles to join. The reason joining bubbles is bad is based on surface area. Multiple small bubbles have more surface area than fewer larger bubbles. The surface area is where the binding takes place.

[Puffers]

I think that it is this first area, where all of these considerations are taking place, and I wonder if "we" are failing to consider the importance of the "second area", and the "dwell time" that is occuring there.

This was along my train of thought in addition this is where I think the whole turbulence issue comes into play. In many of our current designs the bubbles are moving at a very fast and turbulent rate and seem to finally setting or slow down when they rise up the neck. Perhaps you are right Barryhc in that this area is somewhat self-regulating.

perhaps we should concentrate more on detritus skimmers than protein skimmers (protein skimming would be a secondary effect of the detritus skimmer)? Or is there a better way to get the larger stuff out than oversaturated wet skimming?

I will be adding a large mechanical small micron filter to get a lot of that detritus out. Yes it's a pain to remove the floss every few days but well worth it in the long run.

However what we would like would be a "flosses" filter that captures detritus, large particles and possible proteins. Some thoughts on this are instead of having one skimmer, lets make it more than one. Lets create two or three "different" types of "BB" oriented skimmers.

Could there by any way to use buoyancy inside the body more effective? Once most of the particles are in suspension is there any way to ensure once they get sucked into the body (say skimmer) to force them up/out 99% of the time? Perhaps this is where more current, direction and bursts of air and wet neck design would be more effective.

[tinygiants]

A skimmer is already pretty good at getting detritus out of the water. If you are wanting to filter it out, I would filter the skimmer output. This way you are giving the skimmer a chance to remove the detritus from the water column (the skimmate cup is not part of the water column).
Any filter will only change the detritus location, but not its presence in the water column. That is why changing the filter floss is so important.

Right now I use my skimmer on unfiltered tank overflow water. The rest of the tank overflow goes to my refugium (unfiltered), UV (unfiltered), and sump (filter sock).

[barryhc]

I really only threw out filter floss, because it's becoming a lost "art", and the Mfgs. would rather "sell us razor blades". It's great stuff if you change everyday, and especially if "shortly after feeding". In any case, whether filtering with whatever, the queston becomes "before or after".

I tend to think "not much difference" here, but filter maintanence, might be improved if it was after. Some, far more educated than me, have stated that some protiens have an affinity to "solids", and the presence of solids in the skimmer enhances the removal of these special protiens, I don't know.

On the different "areas" and turbulence, my thinking was that the "second area" is not often discussed, or even acknowledged, and it's height woluld be affected by the vertical location of the "top" of a "recirculation path". It could be 1", 3", 8", whatever. It is just an interesting consideration.

> barryhc

[CaptiveReef]

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.

ChemE 120 seconds of dwell time in a skimmer tube would be useless, you are looking to skim the entire tank volume through multiple passes.
120 second dwell time means you are skimming about a gallon an hour,unless you build an 8ft skimmer that at 4inch diameter you stop the flow for 2 mins and let the 5-10 gallons of water in the skimmer tube sit and let it be exposed to the bubbles.That is the only way to get a 120 sec/2 minute dwell time, or a flow rate through the skimmer at an ounce a minute. This is not practical, You need a good flow rate, constant multiple passes will benefit the tank/ system.
Yes your theory will work, but how effective will it be with a fully stocked tank?



CaptiveReef

[Whaledriver]

That is why you want a big skimmer, dwell time of 120 seconds. You can engineer all you want but the bubble has to meet the water. There are cheaper ways of getting a big skimmer

http://www.solar-components.com/protskim.HTM

I also like the idea of using a 5 gallon water bottle and making it into a recirculating skimmer. 4 gallon reaction space with a 2 minute contact time gives you a tank rateing of 120 gallons. This could be as effective as almost any skimmer made by the big names. Remember its just foamy water

[SDguy]

Just thought I'd show some pics of my DIY skimmer, and some tips I've discovered after 3 years of trying different things with it. It's a CC air fed skimmer, except the air is fed into a recirculating needle wheel sedra 5000, instead of airstones. I used airstones many years ago, and just remembered how annoying it was to replace them quite often.

I agree with captivereef. I am skimming my tank with multiple passes. I think the key is to make each of those passes as efficient as possible, but within reason. The most important thing I discovered is to have the tank overflow water be fed directly into the skimmer. So my tank water flows into a large PVC pipe, connected directly to the intake of the skimmer feed pump (mag 5). My sump return pump is also a mag 5, so essentially all the surface water skimmed from the tank goes directly into the skimmer.

For a while I had the flow through the skimmer turned down very low, and this worked well, but it wasn't practical to try to get my sump return pump equal. With tank suface water therefore not all entering the skimmer, I was getting surface gunk (the technical term for it, I believe ) in the sump. Then I tried opening up the flow to the skimmer all the way. What I found was that bubble/contact time in the skimmer actually increased, because the flow would pull the bubbles down somewhat in the skimmer. I could tell by seeing how long it takes for bubble to get to the top of the skimmer when I first turn the air on.

Things I would change if I had to do it again:

1. Use larger than 4" pipe for the body, and put the riser tube on the inside (just for looks). Add another sedra if necessary.

2. Mount the sedra higher up. At it's current height, the water pressure from the column is too much, and it requires airpumps to inject air into the venturi.

Anyways, here are some pics, it's not pretty, but works well:

[Randy Holmes-Farley]

I don't provide any engineering discussion of skimmer design, but I do discuss the chemical mechanisms involved in skimming in this old article:

Protein Skimming: How It Works
http://web.archive.org/web/200103090...skimmers2.html

[CaptiveReef]

Randy excellent article!!!! this really explains the skimmer's function, and how important it is to have a skimmer.
The article gives everybody an idea of what collects in an enclosed system.



CaptiveReef

[jacmyoung]

I don't know if anyone had experimented with injecting colder air into the skimmer? Since solubility decreases at lower temperature, wouldn't that increase the skimmer efficiency by precipitating more pollutants out of water? Of course cold air bubbles can expand in warm water that may defeat the purpose.

But an experiment in that maybe interesting to see. There is plenty of cold air in the winter, and in the summer the air intake maybe connected to the house AC system, who knows it maybe enough to get ride of the chiller?

[barryhc]

Somebody has been thinking, what will come next?

Maybe a "better mousetrap"!

> barryhc

[Puffers]

Since solubility decreases at lower temperature, wouldn't that increase the skimmer efficiency by precipitating more pollutants out of water?

I don't see how it could being that there is less soluble stuff on the surface of the bubbles. I may be missing the boat here but IME the hotter my tank temp gets the more foam I get. So if solubility does increase at a higher temperature, wouldn't it be better to raise the water temp or "bubble temp"?

It also might depend on which soluble items are being taken out at the different temps. Perhaps using the cold air will take out more "desirable" soluble than at a higher temp

[barryhc]

So, "insolubes" may skim better at a lower temperature, and "solubles" at a higher temperature. OK, fine.

You've just about beat it to death here, and even exceptionaly good theory suffers from "chaos" ( as in "Chaos theory" ).

What came first, "the chicken or the egg"? "God, or Darwin"?

Theory in general, only gets you "half way". Imperical data through observation" finishes up", for a new "round" of theorization.

I'm the most theoretical and logical "SOB" that you will ever meet ( or "discuss with" ), check out the "Plenums and wasting" thread for an example.

Practicality "rules", it's a "Question of Balance".

Make a summary of your theories and observations. Incorporate some adjuastability into your design as you finalize it, allowing for further theorizing and observation, based on available adjustments that you design into the prototype.

Yeah, it's "longwinded" and whatever else you prefer, but, this is where you are at.

Go for it, and enjoy the "Journey"!!!!!

> barryhc

[Randy Holmes-Farley]

Like other materials such as inorganic salts, there is not a strict trend toward higher or lower solubility of proteins as temperature is raised. They can go either direction.

Here's a recent article showing one particular set of proteins become less soluble as temperature is raised:

Whey proteins solubility as function of temperature and pH. Pelegrine, D. H. G.; Gasparetto, C. A. Department of Food Engineering, Food Engineering Faculty/UNICAMP, Campinas, Brazil. LWT--Food Science and Technology (2004), Volume Date 2005, 38(1), 77-80.

Abstract

An integrated study was conducted on the effects of temp. and pH on the soly. of whey proteins. The soly. was detd. exptl. in the range of 40-60°C for temp. and 3.5-7.8 for pH. The results showed that, both temp. and pH influenced in the protein soly., and these properties had great interaction. Besides, for whey proteins, the soly. values were min. at the pH value of 4.5, which is the isoelec. point of whey proteins, for all temp. values. It was also obsd. that at pH 4.5, the soly. decreased as the temp. increased, which indicated that the protein denaturation occurred. This behavior was also noticed in the neutrality (pH=6.8).

[jacmyoung]

Originally posted by Puffers
I don't see how it could being that there is less soluble stuff on the surface of the bubbles. I may be missing the boat here but IME the hotter my tank temp gets the more foam I get. So if solubility does increase at a higher temperature, wouldn't it be better to raise the water temp or "bubble temp"?

It also might depend on which soluble items are being taken out at the different temps. Perhaps using the cold air will take out more "desirable" soluble than at a higher temp

Well then have you experimented with locally heating the water in the skimmer to increase the skmimmer efficiency or do you also find this question laughable? Not a difficult thing to try. Had I have a small enough heater to fit in my skmmer chamber I woudl have experimented already. By simply move the heater from after the skimmer to right before the skimmer, maybe enough to observe any change in foam generation.

It is quite obvious most oils desolve better in hot water than cold water. You can skimm a lot of oils by simply cool the water and allow oil to separate from water and float on the surface. Of course that requires extreme temperature change which may not be practical in reef setting.

[barryhc]

A "whole lot more" has been considered in this thread, already, than is considered in most "commercial designs".

This thread has been pursuing "functionality and effeciency" of operation, whereas "commercial designs", primarily pursue "effeciency of sale and profit".

You are "way ahead" here already.

> barryhc