limiting factor for airstone skimmer

[xxbrianxx]

I'm building my own air powered skimmer and I was wondering what the limiting factor is. Is it the size of the air pump and the air being pushed through the airstone? Or does the size of and number of airstones limit it somehow? I would imagine it would be the first, since it wouldn't really matter how large the stone is, the same amount of air is still being pushed through it. Any thoughts?

[lizardfish]

they are good if flow is forced thru them

[rklontz]

You dont really force flow through them, contact time with the air is the name of the game with airstone skimmers. The amount of flow needs match the amount of air, skimmer diameter and the height of the skimmer. My old 6' tall 4" diameter skimmer used a 130gph pump and I used a Tetra Luft air pump through a air diffuser that I made and it managed a 90 gallon overstocked system fine.

 

To answer your question the only answer I can give is "yes". All of those things factor into the limiting factor of airstone skimmers. I hate to direct you to other sites but there was some serious conversations about air skimmers a couple of years ago on reefcentral that you might be able to search for and pull up.

[blasterman]

I've built a lot of airstone skimmers over the years, and over all I've had good luck with them. Cut the bottom out of a 2-litre coke bottle and put a right angle fitting on top. Find some way to adjust the level of the bottle compared to the water line and you have a pretty darn good skimmer once you get it leveled right. I'll certainly take on of those over my old Remora or Sea Clone or Prism any day.

 

For *any* skimmer to work you generally have the same requirements. Dense foam creation with fine bubbles and as much 'dwell' time as possible. Wooden air stones with a 3-4watt air pump produce more foam than pretty much any venturi based or needle wheel skimmer under $200. They typically don't have the dwell time though given that venturi's and needle wheel skimmers start further upstream, so that's where the 'bubblers' lose efficiency. The big reef octi and other high end skimmers are typically *huge* and have immense foam / water dwell times due to their large size. So, they are super efficient.

 

You want a good air pump with a airstone skimmer though because the wooden ones have a lot of back pressure.

[xxbrianxx]

Alright so the consensus is pretty much just contact time? I'm not looking to make a giant 6' tall skimmer though lol. Can I make up for loss in efficiency(contact time) with more bubbles and still get the same relative performance?

[TheUnfocusedOne]

Alright so the consensus is pretty much just contact time? I'm not looking to make a giant 6' tall skimmer though lol. Can I make up for loss in efficiency(contact time) with more bubbles and still get the same relative performance?

 

Smaller bubbles will increase the total surface area exposed over the same period of time. That's were the limiting factor for airstones come in a lot of time, since they don't produce that small of bubbles.

[xxbrianxx]

the bubbles that come out of limewood airstones are pretty dang small...at least visually it seems like they're smaller than the bubbles that come out of most venturi's I've seen.

[TheUnfocusedOne]

the bubbles that come out of limewood airstones are pretty dang small...at least visually it seems like they're smaller than the bubbles that come out of most venturi's I've seen.

 

Then increase volume of bubbles.

[Deckoz2302]

When a skimmer works, it creates bubbles. The bubbles or oxygen separate the proteins/organics from the water and the particle gets sucked inside of the bubble and rises to the top due to the surface tension of the bubble, same reason if you don't have enough flow the organics create a layer on top of the water that looks oily because the force isn't great enough to break the surface tension. Anyway all these bubbles once the organics rise to the top compound on one another which enable the bubbles to become larger creating a foam. The problem with airstone skimmers is one setting the water level and two having enough upward thrust in the bubbles to lift heavier compounded organic foam. The next problem is how do you keep that poorly thrusted foam at the top instead of mixing back into the water when new bubbles rise that are lighter then the heavier compounded foam. The way that is avoided is utilizing a high flow air pump, or with the use of an open ended pump. For example if you took a cheap aquaclear 10 powerhead and removed the propeller cover and placed it under the tube of the skimmer you would get an ample ammount of lift without directly pumping water upward toward the foam. Just something to think about. Sorry if I stated what was already known

[emanliame]

Alright so the consensus is pretty much just contact time? I'm not looking to make a giant 6' tall skimmer though lol. Can I make up for loss in efficiency(contact time) with more bubbles and still get the same relative performance?

 

I don't think an increase in bubble volume will properly compensate for a reduction in dwell time. Surface area (relative amount of bubbles in a given area) increases the odds that the organic molecules you're trying to skim out of the water will make contact with a bubble - that's good. But, there's a certain amount of contact time that's required for various types of organic molecules to properly interface with the bubbles. Some organics will do it quickly, some will take a longer amount of time. If you don't provide enough contact time, you won't remove the organics that require a longer time, no matter how many bubbles you're producing and no matter what their size.

 

With that said, I've never heard of conclusive data around what sort of contact time is optimal and it's not always true that more time is always better. I've also read that if the bubbles hang in the water column too long, previously associated organics can start to disassociate from bubbles and re-enter the water.

[rklontz]

Here is a link to a rather informative article I suggest reading,

http://www.hawkfish.org/snailman/skimmer101.htm

[blasterman]

Good conversation - we need more discussions like this given the fact most cheap skimmers are junk and we have some bad commercial designs out there that can be fixed.

 

Fact of simple physics (or is it chemistry in this case?) is that big skimmers work better than smaller ones. Bigger skimmers typically have much greater water flow meaning their venturi's / injector's work much better. Most of the smaller water pump driven skimmers (typically HOB) I see don't produce much bubble density, and the bubbles they produce are coarse. Bigger skimmers are then typically much taller so there's more reaction time.

 

If you look at 'pro grade' skimmers behind big SPS tanks another thing you'll notice (besides their size) is that a significant portion of the foam column is fairly stationary and just rises slowly. This allows time for water molecules to drain away from protein molecules, and the resulting foam is much dryer and denser than smaller skimmers, aka dark syrup -vs- dark tea in the collection cup (cough Remora cough). With my air stone skimmer I produce as much foam as the big boys, maybe more, and my bubble size is likely much smaller.

 

The problem though is my reaction area is constantly turbulent and the force of water rising with the foam column doesn't allow settle time at the top of the foam column. What I need is a way to slow down the force of water rising without disrupting the bubble flow, but again I'm constrained by the space. I'm going to try wrapping something around the wood-stone to try and produce a slowly rising and wider diameter column of bubbles and see if that helps. Large, expensive skimmers all seem to take into account the need for a low turbulent reaction area.

 

One skimmer design that *might* scale down more efficiently are down drafts. They are tremendously efficient, and you only need a pump with a few hundred GPH to get an injector effect. Not sure if anybody has tried to build a small one.

[rklontz]

Downdrafts require high pressure with alot of volume to get the foam head to work properly. If you under drive a becket it will not work properly.

 

For your airstone skimmer you might want to try and adjust the amount of air going through the airstone. In my experience if you push to much air through an airstone it seems to "pulse". Slowing the flow through the skimmer might help also.

[blasterman]

In regards to a downdraft, I see the issues present with using a Becket injector and I've tried one in the past. They require a freakin' garden hose worth of water flow to run properly, so the entire contraption scales quite big accordingly. The down drafts I've seen that use strictly water pressure to produce bubbles without a Becket are 1000gph and bigger.

 

I thought about a down draft using a Remora type injection, but like you said you need a very strong current to pull the bubble stream around the 'U', and that simply wouldn't work with an open loop injector. Just not enough pressure to force the air stream around with consistency. Then again I'm not a fan of Remora's.

 

What I'm thinking is that you could use a standard 200-300gph power head with needle wheel or venturi instead of the Becket. A big problem with most mid and smaller size skimmers is the pump is located at the bottom of the unit, which means the pump has to fight with water pressure to pull air down. A submerged pump 12" below the water line loses a significant amount of air transfer with a venturi / needle wheel, unless the pump is really big, which scales up our entire skimmer yet again.

 

If the pump were located near the top of the unit, say a few inches below the water line and firing downward all you'd need is 200-300 gph and it should work and deliver just as much foam as a pump much larger in size. Priming it would be a bit tricky because the unit would have to be filled with water, but it should solve the problem.

 

With my air stone rig I'm going to play with using some type of baffling material in the column (spare bioballs, etc) and see what happens. A basic two liter bottle works, but it produces fairly wet skimmate. Slowing things down at the top pf the reaction area might help this.

[rklontz]

It sounds like you are describing a Precision Marine HOT 1 or something akin to it. They nomally use a maxijet 900-1200 or a rio 800 with a venturi mounted before the impeller to help chop up the bubbles. Knowing whether you are making a hang on or a sump type skimmer it is kind of hard to offer suggestions.

 

I have always found Remora's to be rather fickle. It seems that either they work and work well or they dont there doesnt seem to be any consistency or middle ground to them. A dealer once told me it was because they are hand made and it just depends on who made them not sure if that is true or not but it would make sense.

 

For the airstone rig you might want to try a bubble plate above the airstone to help straighten/even out the flow

[Deckoz2302]

One person mentioned the fact that small skimmers normally have they're pumps deep underwater and the pump has to fight to collect air for the venturi. This is true. And a way to solve that is an air pump on the venturi and a few weeks to tune it. When a venturi skimmer receives less air, the pump pumps more water and the water level in the skimmer body rises. When it has more air the water level lowers and there is more space in the skimmer body for the foam head and the water to trickle down and separate from the proteins. In a nano skimmer(under 8inches tall) skimmer body real estate for foam is everything.

 

For example. In my aquatic life nano 115 I have the water height as low as it will go, and also have the output of the skimmer slightly above water. In the 7 inch chamber, almost 4 inches are foam. Also to aid the pumps venturi from sucking for air, I hooked up a small air pump with a bleeder/adjustment valve and spent some weeks tuning. Now I get skimmate like this

 

 

P.s. the foam on top the cup cap almost looks like a nicely rounded set of tata's haha

 

Just my opinion, but in a nano skimmer, (ima pull a beeraholic line) a nice foam head is everything. You want more foam then water in the skimmer body, not by much but id say 55% foam, 45% water. As our tanks are small, so must out skimmers, however molecules don't change size based on our tanks. If you've ever looked at bigger skimmers most of the skimmer body is foam with a little water. So, foam real estate vs water in skimmer body, lift from more air, and time to tune your water height & air injection. Whatever you build you it will be almost a month before you have it perfect. Give two weeks to slime coat/break in, then tuning time. Good luck sir

[al404]

i was reading this because i already have a tunze 9001 but i just sow a video od a mame skimmer

i was wondering is tunze is better or not and a DIY mame could use a CO2 glass diffusor instead wood

[Markburns43]

It's worth a try but I think maybe the co2 diffuser might be two fine and and an air pump might not be able to provide the right pressure to make it diffuse nice. Right now the wood air stone is probably your best bet.