Foam Generators--Skimmer Designs

[Lavoisier]

For those that figure one is better than the other:

http://www.advancedaquarist.com/2009/1/aafeature2

Offered FWIW.

Well, I made the mistake of reading this great article this evening as I was quaffing a couple of beers. I don't know if my head is spinning because of the brews or the article!!

In any event, I am leaning toward the downdraft, airstone design because of both intial cost as well as ongoing cost. I can gravity feed the skimmer to achieve 2x tank volume per day (so I save upfront cost and ongoing kWh or a water pump) and a high quality air pump is about the same as a recirculating pump and a third the cost of a quality pinwheel pump.

However, what about noise in relation to an air pump? Sound (more explicitly the lack thereof) is important to me (I will use a Bean overflow/return for blessed quiet), so I am wondering if any of you (in the distant past) have used a downdraft/air stone skimmer and moved away from this design because of the decibel levels?

Also, I cannot find in any of the literature the size (cuin/min or psi) of a pump needed for a 9"-10" diameter/60" h skimmer. Any thought or experience would be greatly appreciated?

 

[Lavoisier]

I'm sure I'll get flamed for this but here goes...

A penductor is claimed to add 4x the volume of the flow of the surrounding liquid to the pressurized fluid when submerged (4+1=5... Hence a 5x multiplier). If the surrounding volume is gas (air), I would expect that the volume of injected air would be less since air is compressible while water is not... How much air? I expect it would depend on the pressure of the injected fluid... I would have to research that but that should answer the question of injected air volume.

The best forums I have found are based on firefighter hoses since these kinds of penductors are used to add foam to the pressurized nozzle outputs.

Karim, I would welcome any research you discover concerning "injected air volume," though I am leaning toward counter current/airstone I am concerned about noise.

I will spend some time on the hose forums!

 

[mussel and hate]

Air stone skimmers are the quietest of the bunch. The airpump can be in the next room or whatever sort of soundproofed enclosure you dream up.

Air stone skimmers are add less heat to the water than needle wheel or venturi, particularly if it's fed from the overflow drain via gravity.

Air stone skimmers just aren't new, shiny and sexy.

 

[RocketEngineer]

while researching, I found this picture and thread in the malaysia marine club

<a href="http://s1062.beta.photobucket.com/user/karimwassef/library/" target="_blank"><img src="http://i1062.photobucket.com/albums/t496/karimwassef/730_penductor_air_zpsd9274721.jpg" border="0" alt="Photobucket"/></a>

The penductor injects water and air even when completely submerged... much like a needlewheel I expect.

http://www.chuisui.com/viewthread.php?fid=6&tid=8809&action=printable

The original article: TOTM November 2005

Revisit: TOTM April 2008

For clarification, the penductors play no direct role in adding air to the system. Instead, air pumps fill a suction chamber which is fed into the return pumps. The air pumps are controlled by the AquaController 3. The introduction of air occurs once every three hours for one minute.

On the original topic, I think der_wille_zur_macht hit it right on the head. Airstones, venture/beckett and needle wheel skimmers all work and according to everything I have read, none work "better" than any other. Instead you have to choose between the pros and cons of each. "Better, Faster, Cheaper; Pick 2" comes to mind. Airstones make fine bubbles but the stones can degrade over time and need replacement. Beckett skimmers require a pressure rated pump that can draw lots of electricity. Needle wheel skimmers have a nasty tendency to collect items within the pins. In all three styles, the way to improve the skimmer is by having smaller bubbles and longer contact time. Counter-current designs and larger reaction chambers add contact time in two different ways but one requires a lot of height while the other takes up a lot of space in a stand or sump. Smaller bubbles are achieved differently with each style of skimmer but the best results are a skimmer body full of white foam.

At the end of the day deciding on any of these three styles is a matter of compromising. Living with one compromise over another is why there are so many different options out there to purchase.

RocketEngineer

 

[karimwassef]

Thanks Rocket. I couldn't trace the origins of that post and couldn't find an explanation for the bubbles. It looks like the use of a penductor could be comparable to a Beckett with an air dial to control the injected gas?

Lavoisier,

My first DIY skimmer was a countercurrent airstone... it's just so easy to make. My experience was that the best "fine-bubble" airstones degrade very quickly and needed replacement every few months to maintain performance. You could let it go, but the skimmate would gradually turn wet and then fail altogether.

If efficiency is of greatest value, how about a downdraft directly from the main tank drain (assuming you have enough room under the tank)? This way, you don't need any pumps at all, just a penductor on the inlet with air and maybe an airstone for good measure at the bottom? This would be a near passive skimmer? I haven't put any thought into it, just an rough concept.

 

[karimwassef]

In terms of using a pump for the beckett style injection for my 10 foot skimmer (8" diameter), I was looking at a Reeflo snapper which would consume 100W with nearly no head (inlet and outlet at nearly the same level) for foaming and push 2500gph. Another snapper can also be used to push water through the skimmer with a downdraft inlet at the top consuming 110W at 1750gph with a 6 foot head.

http://www.marinedepot.com/pumps_md...ead_manta_ray_tiger_shark_information-ap.html

So.. ~210W total for both pushing and foaming. Makes sense? Is there a better way to get the same effect?

 

[Orm Embar]

I remember using a 3" diameter AMiracle HOB skimmer on my first reef tank with a Tetra LuftPump. It had nice bubble size and density, and I liked it, but my problems with it were 1) replacing the limewood airstones, as the ceramic ones didn't have nearly as fine bubbles, and 2) the hum of the Tetra LuftPump.

I have to laugh at the Advanced Aquarist article - the evidence supports the heretical idea that all skimmers work, and that the latest and greatest is more about getting your money than about something that works.

I would think that a Beckett would cost more to run in the long run due to electrical costs for a more powerful pump, and that airstone skimmers would simply be a pain to keep replacing the limewood airstones. As you can tell, I like the idea of needlewheel skimmers (and remember when the Euro-Reefs came out); I had a downdraft skimmer at the time (ETS Reef Devil mini) running off of a Mag (I think) and just took up a ton of space in my sump.

I did read an article a while back about using a countercurrent airstone skimmer filled with bioballs and a ceramic airstone, with the larger bubbles from the airstone being reduced to smaller sizes by hitting the bioballs, thus simulating a limewood airstone but not needing constant replacement. Also would have less turbulent flow in the skimmer and presumably a more stable air/water interface and foam column, in theory.

FWIW.

 

[der_wille_zur_macht]

Downdraft and beckett skimmers require pressure to run well - I don't think a gravity feed from the DT would work (unless it's several stories above the skimmer at which point you're paying the energy price with your return pump anyways) and the Reeflo snapper konks out at a mere 11 feet of head so it probably won't be ideal, either - typical pumps for beckett skimmers are usually able to push 20+ feet of head.

Look at the sizing guidelines for eductors here, they're all based on pressure:

http://www.kthsales.com/website/Misc/hello_salt_water_enthusiasts.htm

 

[RocketEngineer]

Thanks Rocket. I couldn't trace the origins of that post and couldn't find an explanation for the bubbles. It looks like the use of a penductor could be comparable to a Beckett with an air dial to control the injected gas?

If efficiency is of greatest value, how about a downdraft directly from the main tank drain (assuming you have enough room under the tank)? This way, you don't need any pumps at all, just a penductor on the inlet with air and maybe an airstone for good measure at the bottom? This would be a near passive skimmer? I haven't put any thought into it, just an rough concept.

In terms of using a pump for the beckett style injection for my 10 foot skimmer (8" diameter), I was looking at a Reeflo snapper which would consume 100W with nearly no head (inlet and outlet at nearly the same level) for foaming and push 2500gph. Another snapper can also be used to push water through the skimmer with a downdraft inlet at the top consuming 110W at 1750gph with a 6 foot head.

http://www.marinedepot.com/pumps_md...ead_manta_ray_tiger_shark_information-ap.html

So.. ~210W total for both pushing and foaming. Makes sense? Is there a better way to get the same effect?

Downdraft and beckett skimmers require pressure to run well - I don't think a gravity feed from the DT would work (unless it's several stories above the skimmer at which point you're paying the energy price with your return pump anyways) and the Reeflo snapper konks out at a mere 11 feet of head so it probably won't be ideal, either - typical pumps for beckett skimmers are usually able to push 20+ feet of head.

Look at the sizing guidelines for eductors here, they're all based on pressure:

http://www.kthsales.com/website/Misc/hello_salt_water_enthusiasts.htm

karimwassef, As soon as you posted that image I knew where it came from. I like the idea of turning the ENTIRE TANK into a giant skimmer for a short span of time. It mimics what waves do by infusing lots of air into the water and creating the sea foam found on beaches.

Like der_wille_zur_macht said both penductors and becketts need pressure rated pumps to run properly. When talking about efficiency, pressure rated pumps are anything but efficient at moving water. When it comes to skimmers, there are more energy efficient ways of producing bubbles, most notably an air pump and pin wheel.

As an example: A Reeflo Dart with the impeller changed out for a Pin Impeller.

What you get is a Reeflo Orca combined with a Alita AL40 air pump which pushes 120 scfh at a total of 181W (135+46). Which isn't bad for any skimmer, especially a large one.


der_wille_zur_macht, I think karimwassef has the terminology confused. If you had the drain line from the tank going to the inlet of the counter-current flow, all the water from the drain would be going past the bubbles created by a recirculating . As long as the skimmer body was big enough to handle that flow rate it would make sense but high flow through a skimmer is counterproductive.

RocketEngineer

 

[Dave Thebrewguy]

I remember using a 3" diameter AMiracle HOB skimmer on my first reef tank with a Tetra LuftPump. It had nice bubble size and density, and I liked it, but my problems with it were 1) replacing the limewood airstones, as the ceramic ones didn't have nearly as fine bubbles, and 2) the hum of the Tetra LuftPump.

I still have mine. I think I paid around $130 for it 20-25 years ago and it's still in use on my lightly loaded 90 gal. I'd say I got my moneys worth out of it but when I consider replacing it, it always comes down to "if it ain't broke, don't fix it." I just rebuilt the Hagen Optima that powers it and I have a spare lime wood air stone, so I guess I'll be hanging on to it a while longer.

 

[karimwassef]

Ok.. so how about an Iwaki MD55RLT (1000gph) with 17psi (27ft head). That's 180W for the foaming and should be within the window of use for the penductor.

For flow through, an Iwaki MD20RLXT (540gph) at 60W.

The total is ~240W and with ~1/2 the flow I was originally expecting. It's not as efficient as the 180W reeflo orca+air but I'm not convinced that the quality of the skimming with a needlewheel is comparable. We've already concluded that it's buyer's opinion and I have not seen a needlewheel with the bubble density or size from a beckett.

The idea I called downdraft is what Rocket described... use the flow from the tank to the sump against a bubble column... nearly free skimming.

 

[Lavoisier]

Downdraft and beckett skimmers require pressure to run well - I don't think a gravity feed from the DT would work (unless it's several stories above the skimmer at which point you're paying the energy price with your return pump anyways) and the Reeflo snapper konks out at a mere 11 feet of head so it probably won't be ideal, either - typical pumps for beckett skimmers are usually able to push 20+ feet of head.

Look at the sizing guidelines for eductors here, they're all based on pressure:

http://www.kthsales.com/website/Misc/hello_salt_water_enthusiasts.htm

Wille, I want to respond to your thoughts here with my understanding of Escobal's chapters on Skimmers (I think you are partially referring to my idea of using a gravity feed to my counter current/air stone skimmer). I am concentrating on his summary statements and graphs because I often get lost trying to run all his formulas. If I am missing something as a result, I am hoping someone will point it out (flame!).

Escobal states that effective skimming includes both a mechanical and a chemical component and he expresses their relationship as Bombardment Rate (Ration) R, in the formula Rb = tw(water)/ta(air) and t is dwell time. The higher the Rb is the better. So slow water movement and rapid air movement tend to produce a higher Rb. He creates a chart with an Rb of 10. I can turn over my 500g twice a day with a flow rate of only 380gph. I can achieve this rate with ¾" pipe and a 5" head with open discharge into the skimmer (courtesy of BeanAnimal's hydraulic page).

So far so good (I think!). The piece that I simply cannot work through his Required Air flow rate at the end of the second chapter. To get the requisite "œbloom" of air bubbles in a 9" 5ft skimmer I believe I will need a pump that provides 7.0 psi against water pressure. Does that sound in the ball park?

 

[der_wille_zur_macht]

If you know the depth the airstones will be at in the skimmer body, you can calulate the pressure the pump will have to overcome - 1 foot of head is .43 psi. So your 5' tall skimmer, worst case (airstones at the very bottom and full to the brim) will have 2.17 psi of pressure against the air pump. So, if you know your desired air flow rate you can look for a pump that will do that flow rate at 2.17 psi. Up above someone mentioned the Alita air pumps which are kind of a gold standard in high-flow air pumps IME. The page linked has a performance curve for the whole product line. Say you needed 45 lpm of air - about 95 scfh - the Alita AL40 would be perfect.

It might be worth noting that the design process for DIY'd "modern" skimmers often focuses on matching the neck diameter to the expected air flow rate. This is something I don't have intimate knowledge of in terms of being abble to lay out the math but you might want to include it in your research. It gets a bit into the gray area we discussed earlier of sifting out actual performance enhancement from marketing hype, but it's pretty clear that neck design and size plays a role in overall skimmer performance. Too big of a neck or a poor transition and the foam breaks down before it builds up enough to make it to the collection cup. Too small of a neck and the skimmer turns into a foam cannon, where the airflow up through the foam is too strong for stable skimmate production (this is definitely amusing to see in person though!)

 

[Lavoisier]

Thanks, wille, this is very helpful and just what I needed. I think I will build a counter current/air stone skimmer and try it out. If I find it is too much ongoing work to tune and replace air stones I can always convert it a foam nozzel (Beckett like) skimmer.

 

[Lavoisier]

Like der_wille_zur_macht said both penductors and becketts need pressure rated pumps to run properly. When talking about efficiency, pressure rated pumps are anything but efficient at moving water. When it comes to skimmers, there are more energy efficient ways of producing bubbles, most notably an air pump and pin wheel.

As an example: A Reeflo Dart with the impeller changed out for a Pin Impeller.

What you get is a Reeflo Orca combined with a Alita AL40 air pump which pushes 120 scfh at a total of 181W (135+46). Which isn't bad for any skimmer, especially a large one.

RocketEngineer

I do have another question that has been bothering me for awhile. Escobal claims that theoretically no more than 13% of the skimmer chamber can be air without the bubbles merging and forming larger ones, reducing the total surface and decreasing mechanical and chemical efficiency. Would an injector or pinwheel skimmer put more than 13% air volume in the chamber? (which is what my lay mind would think)

From the articles cited earlier those skimmers are equally effective as a counter current/air stone so either one of several things may be happening??

1.They are not adding more than 13% and Escobal' theory is correct.

2. They are adding more than 13% and Escobal's theory is incorrect.

3. They are adding more than 13% but are doing it in such a way (an advance in technology) that the bubbles are not combining.

Not sure why this is bothering me but it is!

 

[der_wille_zur_macht]

I honestly don't have any idea but I bet you could determine which was likely with some math based on flow rates for typical skimmer pumps. I also don't know what Escobar based his analysis on or what his criteria were so again it's hard to know if the 13% number is even valid.

Off the cuff, I think it would probably be important to consider that most modern skimmers have a pretty high flow rate per unit of volume compared to his designs, so if at first it seems like they're flowing more air, you might want to remember that they're flowing more water as well.