Skimming Principles

but tinygiants did find a flaw in my calc, that the corresponding pipe/box size with the water flow. I understand bernoulli's principal, and the reynolds number, but for the life of me I cant seem to get it to work in excel.

taken a tube size of 4 inches X 10 feet with a set pump rate, then and increasing or decreasing the tube diameter and lengths should affect the dwell due increased/decreased flow rate but this isn't in my calcs, and im unsure how to apply this number so it appears correctly, and even more so how do I make it a function in excel?
 
crazzyreefer said:
dwell time and air ratio are separate issues. a bubble doesn't need 20 min, only 120 seconds, in that 120 seconds it has fully formed and and the organics and oils have attached, at this point it is unlikely to burst in the foam chamber, you couldn't have dwell time of 20 min if you wanted to! the reasoning behind the saturation point is the air is the filter media, so the less air ratio the less foam, the less filtering, above the saturation point of 13% the bubbles are no longer stable and join with others, causing belching in the foam tube and larger bubbles offer less surface space for the organics to adhere to. Escobal has decided that a ratio of 10% is perfect, but the actual number is really 13%.

Randy,ChemE, thank you.... I kinda knew that, but I was hoping that I was wrong. So it will remain an unknown, and CFM input would have to remain adjustable. or is so small of a number that is not important.
Click to expand...

You and I are not on the same page here.

The 20 minute dwell is the water dwell not the air dwell. That is based on bombardment rate which is a ratio of water dwell:air dwell. So if an air bubble dwells for 120 seconds, the water needs to dwell for 1200 seconds to meet the ratio. Again I question the validity of this ratio. Is a ratio of 10 better than a ratio 20? Why?

Escobal did say that 13% is the correct volume of air bubbles (On page 1 I copied the quote).

As I said before this is all very stimulating conversation. I really enjoy all the though this discussion is promoting.

Dale
 
tinygiants said:
"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.
Click to expand...

Here is where I found the 13% air volume goal.

Dale
 
Ok I re=read the entire article... no water dwell was mentioned...or do I need glasses... the water dwell you may be taking out of context ... you dont need water to sit in the skimmer for 20 min, as long as the bubble had non turbulant contact for at least 120 seconds, now the turn over rate is 2-3 times of system volume not just tank volume.
 
You are right, that article mentioned the air volume. I got the bombardment rate (takes into account water dwell and air dwell) from 2 other websites (check your PM). One site was a discussion board that stated the goal of 10. The 2nd was a article that copied a tab le from Escobals book. In that table it was the 2nd law, but it did not stae what the goal ratio should be. It merely stated it was important.

In previous post I question the validity of the ratio. In fact that very question was the basis for post #1. I could not get that goal with my intended design, and I question how important it was.

Dale
 
Here is the start of a file to figure bubble dwell in a verticle skimmer assuming the 1.17 in/s from earlier post.

Dale


This file is wrong. New file is below.
 
Last edited:
Those numbers were just what I had at the time of saving. Total volume is calculated in line B14.

There was an error in the bubble calcs of the original sheet. Use the newer version.
Dale
 
the bubble rise is at 1.17 or 3cm Clean... 2cm when its bonded with organics, so it throws another wrench in the mix, this is the whole piont, we dont want just clean bubbles to rise, so the 120 rule is also out the window... sorry, now the bubble still needs 120 seconds, but as it collects organics and oils, it will slow down, so if its a counter current model and it it slows down 1/3 of the rise rate the bubble will not enter the foam area.
 
In my modeling I am getting a water flow of .6 in/s. At a bubble rise rate of 2cm/s = .78 in/s. That would equate to corrected rise rate of .18 in/s

Why would this bubble (organics and all) not continue to rise?

Dale
 
Counter current skimmers usually have the inlet near the top but not above water line. The water will be counter current below the inlet and less directional above the inlet. The water speed will have less effect on the bubble at this point. So the bubble rise rate will slow down as it picks up organics and its overall rise rate will increase as it leaves the area of counter current influence.

If the spacing is worked out you could still have your cake and eat it too.

Dale
 
yes it would and it would also collectivly add to the Air ratio bumping it up past the 13% mark... ok lets think this through, you get the skimmer set up, your water isnt as clean and clear, so lets say your bubble rise rate is vary slow, and when you feed, even more organics enter the chamber, the air ratio goes through the roof... then it settles down again... the object isnt to have the fluctuation and a constant skimming, so how do you adjust air Automaticly? DO controller? redox Controller Orp controller? and how do you controll the air in the first place? adjusting the valve on a pump? can the funtion be done automaticly? cheaply?
 
I think the rise rate difference is really going to be insignificant compared to the other parameters.

The bubble rise is going to slow down on a curve. The bubbles at the top will be slower due to the load.

Skimmers by nature will not skim if the water is clean no matter how long the bubble sits. When we feed or medicate we are going to get skimmate.

I think I would sacrifice the efficiency for the time peroid that the skimmer recovers from feeding. I would accept slightly over 13% bubbles. We routinly accept < 13%. In fact i am not sure the average hobbyist has the ability to measure 13% bubble volume.

Dale
 
but the curve adjusts to the water quality... and a poor designed skimmer wont clean clean water... but made properly it will! and its at this end that a new skimmer principals might make all the difference in other parameters, we just dont know... its not tried or proven... foam area cleanliness is key to foam removal, if the skimmate breaks, it coat the side walls and becomes less efficient. lets have our cake...and eat it too...just come up with a way to adjust a air stone...could the skimmer have a float valve that operated on the buoyancy like a hydrometer and it controlled a air valve?
 
ok I read the article that you got the bombardment rate... and I believe escobal is flat out wrong on rule 3, the rest are accurate, but skimmer height does have an effect of bombardment rate , and rule 2 I just question... I bet the rules are meant to be simplistic, but at that point they are out of context. and no longer valid. again this is his theory, and can easy be disproved, he states a clean bubble rise only increases contact time not bombardment rate... contact time is bombardment... you can have a clean bubble rise without it further being bombarded! I wondered why you were confused... its rules are in a circle. and are contradicting to each other!
 
Couldnt you measure a 13%bubble volume by the weight of the chamber before and with the foam? Just put a electronic bathroom scale under the skimmer and do the math.

This might help in tuneing and designing a recirculating skimmer. You can design it in therory but the variables change too much to predict it cost effectivly.
 
yes I can do it by weight! and a electonic model colud control a air valve!, ok I need my friends to start taking apart their scales... like they are not in enough trouble....


but this is compleatly workable, if not a bathroom model, a smaller electonic one.

ok water weights 8.33 lbs at 87% = 7.2471 Lbs

Thank you, this will work!
 
Have you seen the DC motor on a needle wheel the Bubble King guy is testing? Very efficient. You could also use it with a speed controler that is regulated by the weight of the water in the chamber. Should be a simple control system with the right computer program.

This would create a skimmer that would warrent a premium price as value added product. Some other premium skimmers dont seem to warrent the premium
 
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