DIY High Efficiency RG CO2 Reactor - The Planted Tank Forum
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post #1 of 24 (permalink) Old 02-19-2014, 03:42 AM Thread Starter
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DIY High Efficiency RG CO2 Reactor

Hello All,

After studying the designs of the Cerges and RG reactors and evaluating ceramic diffusers and other methods to inject CO2 I decided to build a high efficiency/ high rate CO2 reactor.

This reactor is designed for tanks in excess of 100 gallons (I have a 90 gal and a 200 gal tank-both are supplied by same reactor)

The goals are:

1) Eliminate bubbles from my tank
2) Fast rate of CO2 dissolution in water
3) Quiet operation
4) Self regulating CO2 delivery with low likelihood of overdosing and killing fish

The design is based on a vertical bubble reactor (basic RG concept) with the use of a internal venturi to pull CO2 from the top airpocket into the water column. Tangential injection and counter current flow is used to slow the air bubble rise and increase contact time. Center outlet tube picks up relatively bubble free water to return to the tank.

To control CO2 delivery I installed a small float switch in the top wired to the solenoid on my CO2 tank.If the reactor water level is high then the solenoid is energized and CO2 flows in until the float drops and opens the switch shutting off the CO2 flow.

Next some pictures...
Exploded parts view:

Float switch

First step: assemble the vinyl discharge hose into the 1.5" bushing (silicone or PVC cement can be used but the hose should fit very tightly).

Now -to make the venturi...to make the water jet simply glue in concentric pieces of vinyl tubing inside the 3/4" barb fitting. This will be under pressure so make sure to use silicone or PVC cement. Later on I added an additional piece of tubing inside to reduce the nozzle diameter even more.

The other part of the venturi is the gas inlet-this is made by drilling several holes in the top of the bulkhead fitting. The idea is to have air drawn in at the very top of the housing. Smaller holes produce smaller bubbles. The exact number of holes is not important.

Screw in the barb fitting into the bulkhead fitting to create the venturi assembly. I like using the threaded parts as the barb/nozzle can be unscrewed from the outside and cleaned or repaired if needed.

Finally screw in a 3/4" threaded nipple to complete the venturi.

and add a elbow fitting to create a tangential discharge, i chopped off the end as it would have extended close to the wall of the reactor. On the right you can see the float valve assembled as well. A small plastic barb x 3/8" threaded fitting was also glued in - this is where the Co2 comes in

Now we can assemble the clear PVC to the white PVC sections - using PVC cement

Here is what the complete reactor looks like....

Water is injected from the top, the internal venturi mixes the CO2 in the headspace into the water as microbubbles and swirls it around. Large bubbles float to the top and are redrawn into the venturi. Relatively clear water is picked up from the bottom the reactor and discharged.
Finally after 18 trips to Home Depot - system in operation!!

And some close ups - venturi in action

and the bottom - pretty clear of bubbles

Last edited by rezco; 02-19-2014 at 09:23 PM. Reason: add pics
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post #2 of 24 (permalink) Old 02-19-2014, 04:00 AM
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Efficiency is from 1) pressure and 2) turbulence. Dwell time is a measure of inefficiency since it's a direct indication that dissolution is slow.
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post #3 of 24 (permalink) Old 02-19-2014, 06:08 PM Thread Starter
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Quote:
Originally Posted by Solcielo lawrencia View Post
Efficiency is from 1) pressure and 2) turbulence. Dwell time is a measure of inefficiency since it's a direct indication that dissolution is slow.
Well - I should have been more clear in my post but did not want to get too technical. I guess we can have a different interpretation of 'efficiency'.

High gas pressure is one way of increasing gas absorption but it consumes more power as the recirculation pump now has to work against a much greater head. I want lower energy consumption and lower CO2 consumption. In this design I am using a 45 watt rated pump which due to to the low head is actually pulling close to 10 watts.

To elaborate - the dissolution of CO2 is rate limited by conversion of dissolved gaseous CO2 to carbonic acid. The actual gas mass transfer into water is quite fast.

Therefore whether a trickle design (like Cerges) or a bubble column (RG type) is employed - surface area will be the rate limiting step. In a Cerges type reactor the gas-liquid surface area is limited by the type of media/canister volume etc so if you increase gas pressure you can get a little more CO2 absorption.

To double the rate of CO2 absorption doubling the surface area is equivalent to doubling the gas pressure. Obviously the latter is much more energy intensive.

In the bubble column approach if the gas bubbles are small enough then the resulting surface area can be quite massive and high pressure is not needed. Ultimately that is what I am trying to achieve here and will share some results.
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post #4 of 24 (permalink) Old 02-19-2014, 06:12 PM
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Next some pictures...
eagerly waiting


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post #5 of 24 (permalink) Old 02-19-2014, 10:10 PM Thread Starter
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Here is the parts list (I will update with costs later)

24 x 4" clear PVC pipe (0.25 in wall thickness)
Toilet Flange Cap PVC 4"(this is smooth with no grroves or lettering so ideal for sealing the bulkhead fitting)
PVC Drain Inspection lid and female fitting 4" (use this on the bottom to be able to open the reactor nd clean it)
1" ID vinyl hose
PVC T 4" x 2"x 4"
PVC Reducer 1.5" x 2"
1/2" barb fitting with 3/4 thread
3" x 3/4 threaded nipple
3/4" threaded elbow

Float switch - 10Watt x 110VAC max

3/4 plastic bulkhead fitting (1" external thread, 3/4" internal thread)

assorted vinyl tubing for venturi

Pump 420 GPH (powerhead fountain pump)
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post #6 of 24 (permalink) Old 02-19-2014, 10:25 PM Thread Starter
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Here is some cost and sourcing information for the key parts:

Clear PVC - bought off EBay ($53 incl shipping). I used 1/4" wall thickness material but 1/8" is sturdy enough and a lot cheaper.
Bulkhead fittings (Ebay 4$ ea)
Float switch (Ebay $3)
Recirculation Pump 420 GPH rated E Bay - $19

One thought on construction - the vinyl tubing for discharge proved to be unnecessarily expensive as it required too many parts. I suggest installing a second flat toilet flange on the bottom and adding a second bulkhead to draw out the water. Much cheaper in parts.
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post #7 of 24 (permalink) Old 02-19-2014, 11:34 PM
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You're still designing around an inefficiency which is why that thing is huge. Greater pressure allows for smaller reactor chambers.
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post #8 of 24 (permalink) Old 02-20-2014, 01:31 AM
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The Dual venturi or even a single venturi design is simpler and will purge the gas build up. This works in both external reactors as well as internal designs. I did those about 15-20 years ago now.

What's weird is that many makers copied the design, but not the venturi to purge the gas.

You can also do this modification with the AM1000 using the purge valve and run the air line from that to the return pump or the canister etc.

http://www.barrreport.com/showthread...al-CO2-reactor

I think a better design is just using a long piece of rigid 3/16" as a bubble counter inside the reactor tube and then have your pre set depth set for the purge gas level build up before the venturi kicks in to atomize and waste/purges the gas out.

There's likely several ways to do this and purge as a pre set level, but this is about the easiest.

A 12-18" x 3" dia tube should be plenty for up to about 200 Gallon tank or so.

I think you can reduce the size of the thing if you use say a 300 gph or thereabouts power head. That's all I use on a 180 Gallon tank.




Regards,
Tom Barr
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post #9 of 24 (permalink) Old 02-20-2014, 04:26 PM Thread Starter
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Quote:
Originally Posted by plantbrain View Post
The Dual venturi or even a single venturi design is simpler and will purge the gas build up. This works in both external reactors as well as internal designs. I did those about 15-20 years ago now.

What's weird is that many makers copied the design, but not the venturi to purge the gas.

You can also do this modification with the AM1000 using the purge valve and run the air line from that to the return pump or the canister etc.

http://www.barrreport.com/showthread...al-CO2-reactor

I think a better design is just using a long piece of rigid 3/16" as a bubble counter inside the reactor tube and then have your pre set depth set for the purge gas level build up before the venturi kicks in to atomize and waste/purges the gas out.

There's likely several ways to do this and purge as a pre set level, but this is about the easiest.

A 12-18" x 3" dia tube should be plenty for up to about 200 Gallon tank or so.

I think you can reduce the size of the thing if you use say a 300 gph or thereabouts power head. That's all I use on a 180 Gallon tank.
Tom- I agree it can be smaller.This time I am looking just to explore some concepts and gather data to support a smaller design. For a 100-200 gal tank I would probably aim for a 2" dia x 24" tube. Making the venturi and tangential nozzle smaller is bit harder though.

Looking at your design-are you using countercurrent flow - CO2 bubbling up and water flowing down? Any media to break up the bubbles and increase contact time?

Can you share some details on water flow rate, CO2 rate and CO2 concentration in the outflow?
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post #10 of 24 (permalink) Old 02-21-2014, 03:28 AM Thread Starter
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How does it perform?

Quote:
Originally Posted by plantbrain View Post

I think a better design is just using a long piece of rigid 3/16" as a bubble counter inside the reactor tube and then have your pre set depth set for the purge gas level build up before the venturi kicks in to atomize and waste/purges the gas out.
I think the waste gas you are referring to is atmospheric (mainly nitrogen) which has very low solubility in water compared to CO2. So in systems where you start up initially with air trapped inside it can take a very long time to purge out. But once you have Co2 it will actually dissolve out into water very quickly.

In this reactor I had about 300 cc of atmospheric air when it was initially turned on. It took about 4-6 hours for it to be completely dissolved in water and eliminated so there is actually no need for a separate purge circuit.

When CO2 is pumped in it dissipates within a few minutes.

To be clear all the gas is completely dissolved in the water there are no microbubbles or mist coming out of the discharge line. The key is circular flow inside the reactor. This pushes big bubbles to the center where they rise up. Microbubbles are on the side and gradually dissipate at the bottom. here is a schematic:




I plumbed the reactor on a closed recirculating loop (about 400 GPH). The loop is interconnected via a T to a sump and another T to a pump. The pump pulls out CO2 rich water from the recirculating loop at 0.5 GPM and sends it to my tanks.

The maximum CO2 airflow that the reactor can fully dissolve is 0.3 LPM (3-5 bps in contrast is about 0.02 LPM)!!

..so its very heavy gas flow that can be successfully dissolved by this reactor. Obviously I run it at only about 3 bps to support 300 gallons of tankage but you can see that with this design its possible to deliver a massive amount of CO2 (far more than is practically required!).

At 3 bps on the recirculating loop I tested the pH of water extracted by the 0.5 GPM pump that's pumping to my two tanks. dKH = 7,pH before CO2 was turned on = 7.8, after 30 minutes of CO2 at 3 bps pH of water delivered from reactor is 6.2 (this was the limit of my API test kit so pH could be lower).

Therefore the CO2 ppm is 90ppm. I am only delivering it at a slow rate to my tanks so the tank ppm is close to 30 ( by drop checker).

I think if I was to increase the flow from the reactor into my tanks the ppm would be even lower but its not required.

I think to be able to adequately delivery CO2 to approx a 300 gallon volume without the 7Up effect is really worth it.

The next step will be to design a more compact version with lower flow. I think I can do that in a 2' x 24 " cylindrical column. If any one is interested in trying one out please let me know.
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post #11 of 24 (permalink) Old 02-21-2014, 04:31 AM
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Quote:
Originally Posted by rezco View Post
Tom- I agree it can be smaller.This time I am looking just to explore some concepts and gather data to support a smaller design. For a 100-200 gal tank I would probably aim for a 2" dia x 24" tube. Making the venturi and tangential nozzle smaller is bit harder though.

Looking at your design-are you using countercurrent flow - CO2 bubbling up and water flowing down? Any media to break up the bubbles and increase contact time?

Can you share some details on water flow rate, CO2 rate and CO2 concentration in the outflow?
Generally it is better to use a fatter tube, say 4x 12" tube vs such a long tall one, dwell time and gravity, reduction of the current, not so much distance traveled is the key there.

This is a trade off but often you want to place these things under a cabinet, so.......a 24" tall whopper.........not so easy, a 12" fatty, pretty easy.

You should try out various rigid tube placements and depths.
If you add say 4-5 of them at various depths, then plug with air line plugs, you can try each one out at a different level and see the results etc pretty easily.

These are the main 2 advantages, fat tube and the venturi placement etc.

The DIY internal CO2 reactor does this also and uses very cheap stuff to make it.




Regards,
Tom Barr
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post #12 of 24 (permalink) Old 02-21-2014, 04:48 AM
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I think the waste gas you are referring to is atmospheric (mainly nitrogen) which has very low solubility in water compared to CO2. So in systems where you start up initially with air trapped inside it can take a very long time to purge out. But once you have Co2 it will actually dissolve out into water very quickly.

In this reactor I had about 300 cc of atmospheric air when it was initially turned on. It took about 4-6 hours for it to be completely dissolved in water and eliminated so there is actually no need for a separate purge circuit.

When CO2 is pumped in it dissipates within a few minutes.

To be clear all the gas is completely dissolved in the water there are no microbubbles or mist coming out of the discharge line. The key is circular flow inside the reactor. This pushes big bubbles to the center where they rise up. Microbubbles are on the side and gradually dissipate at the bottom. here is a schematic:




I plumbed the reactor on a closed recirculating loop (about 400 GPH). The loop is interconnected via a T to a sump and another T to a pump. The pump pulls out CO2 rich water from the recirculating loop at 0.5 GPM and sends it to my tanks.

The maximum CO2 airflow that the reactor can fully dissolve is 0.3 LPM (3-5 bps in contrast is about 0.02 LPM)!!

..so its very heavy gas flow that can be successfully dissolved by this reactor. Obviously I run it at only about 3 bps to support 300 gallons of tankage but you can see that with this design its possible to deliver a massive amount of CO2 (far more than is practically required!).

At 3 bps on the recirculating loop I tested the pH of water extracted by the 0.5 GPM pump that's pumping to my two tanks. dKH = 7,pH before CO2 was turned on = 7.8, after 30 minutes of CO2 at 3 bps pH of water delivered from reactor is 6.2 (this was the limit of my API test kit so pH could be lower).

Therefore the CO2 ppm is 90ppm. I am only delivering it at a slow rate to my tanks so the tank ppm is close to 30 ( by drop checker).

I think if I was to increase the flow from the reactor into my tanks the ppm would be even lower but its not required.

I think to be able to adequately delivery CO2 to approx a 300 gallon volume without the 7Up effect is really worth it.

The next step will be to design a more compact version with lower flow. I think I can do that in a 2' x 24 " cylindrical column. If any one is interested in trying one out please let me know.
Sounds nice in THEORY, but it's not the case practice, particularly after the about 4-8 hours of running.

I have sumps and CO2 systems that run independent of the return pumps, so if I shut the return pump off, then do my water changes, leave the CO2 running, the pH drops to about 5.3 vs say 6.0 normally in the tank.
The level inside the reactors also goes from maybe 1/2 way down, to the bottom of the reactor tube, roughly 12" or 24" depending on the chamber size.

Is this all N2 gas too? No, the CO2 does NOT dissolve in a linear fashion as you add more, the sump is a small volume and the CO2 builds up, and as it does so, so does the FLUX out of the of the water/sump. So more degassing takes place. It's like running uphill in the sand, 2 steps forward(adding more CO2), 1 step back(more faster rates of degassing).

Fick's 1st law of diffusion predicts this.

These are not sealed systems since the inflow and out flow water is exposed to the air rather quickly. There is also lag times involved.


Observations in real aquariums:
So after my water change, what happens to all that CO2 inside the chamber? In about 1 minute it is all gone.

So why would ANY gas build up if it is NOT CO2?
If you do not add ANY CO2 to a reactor, does it magically fill with gas later in the day? Is CO2 gas we buy very impure? No, it's pretty much 99.90% pure or higher, FDA regulations for beverage grade. So that could be a source.......but why does the level inside my chamber fill so rapidly if the N2 is so hard to dissolve into water?

Something does not add up.

But the new water after the water change is poor in CO2, not rich.

Another way to test this:
You can take the sealed hang on CO2 reactor chamber after several hours of operation, use valves to shut both ends and place it on a different say non CO2 enriched tank, the gas will dissolve rapidly and go into solution. N2 gas does not do that. O2 also does not do that either. You can bleed air into the chamber and try this also. There should be a big difference between the time it takes to dissolve the gas from the build up of CO2 over a few hours vs the same volume of air in the same CO2 reactor.

This is an old issue and I think I'll do this myself to check another way to show this vs the sump thing.

Thank you, gave me a new idea to test it.




Regards,
Tom Barr
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post #13 of 24 (permalink) Old 02-26-2014, 05:17 AM Thread Starter
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Sounds nice in THEORY, but it's not the case practice, particularly after the about 4-8 hours of running.

I have sumps and CO2 systems that run independent of the return pumps, so if I shut the return pump off, then do my water changes, leave the CO2 running, the pH drops to about 5.3 vs say 6.0 normally in the tank.
The level inside the reactors also goes from maybe 1/2 way down, to the bottom of the reactor tube, roughly 12" or 24" depending on the chamber size.

Is this all N2 gas too? No, the CO2 does NOT dissolve in a linear fashion as you add more, the sump is a small volume and the CO2 builds up, and as it does so, so does the FLUX out of the of the water/sump. So more degassing takes place. It's like running uphill in the sand, 2 steps forward(adding more CO2), 1 step back(more faster rates of degassing).

Fick's 1st law of diffusion predicts this.

These are not sealed systems since the inflow and out flow water is exposed to the air rather quickly. There is also lag times involved.


Observations in real aquariums:
So after my water change, what happens to all that CO2 inside the chamber? In about 1 minute it is all gone.

So why would ANY gas build up if it is NOT CO2?
If you do not add ANY CO2 to a reactor, does it magically fill with gas later in the day? Is CO2 gas we buy very impure? No, it's pretty much 99.90% pure or higher, FDA regulations for beverage grade. So that could be a source.......but why does the level inside my chamber fill so rapidly if the N2 is so hard to dissolve into water?

Something does not add up.

But the new water after the water change is poor in CO2, not rich.

Another way to test this:
You can take the sealed hang on CO2 reactor chamber after several hours of operation, use valves to shut both ends and place it on a different say non CO2 enriched tank, the gas will dissolve rapidly and go into solution. N2 gas does not do that. O2 also does not do that either. You can bleed air into the chamber and try this also. There should be a big difference between the time it takes to dissolve the gas from the build up of CO2 over a few hours vs the same volume of air in the same CO2 reactor.

This is an old issue and I think I'll do this myself to check another way to show this vs the sump thing.

Thank you, gave me a new idea to test it.
Hi Tom - my system does run on a closed loop (its the second independent pump that draws CO2 enrighed water out, and allows CO2 poor in).

On initial startup when all the water is CO2 poor the system can dissolve 0.3 Liters per minute of CO2 in the water column (that's way too fast to count in BPS). After a few minutes the water in the loop is saturated and rate of CO2 absorption drops as can be seen by decreasing water level in the reactor as the incoming CO2 displaces the water. The pH of this water is off the chart of my API kit (pale yellow)-so very CO2 rich indeed. It takes just a few minutes to go from pH 7.8 to highly acidic.

When I turn on the second pump to circulate sump water through the reactor the water level in the reactor rises as the CO2 in the head space is consumed, the float switch is engaged and CO2 starts flowing again.

This system is so efficient that the water has to be highly saturated with CO2 before CO2 absorption slows. I have not observed any reduced rate of CO2 absorption when the CO2 is only at 20-30ppm. Very likely its a limitation in the design you might be using - no critcism intended as we are only aiming for 20-30 ppm.

I am going to apply some of my findings to a much more compact design. As you mentioned a fatty 12-20in would be ideal. Right now a 12x3 inch column would work for up to 5 bps at 200 gph water flow.

I am also doing a higher pressure test at around 5 psi on the reactor to make sure there are no small leaks.

Will post here when I have further data to share.

Last edited by rezco; 03-05-2014 at 04:24 PM. Reason: typo
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post #14 of 24 (permalink) Old 03-07-2014, 10:07 PM Thread Starter
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CO2 pressure in reactors

Quote:
Originally Posted by Solcielo lawrencia View Post
Greater pressure allows for smaller reactor chambers.
Here is why the pressure approach is not really applicable to aquaria. There are many publications on CO2 absorption in water. Here is a nice one for reference:

http://www.nist.gov/data/PDFfiles/jpcrd427.pdf and a table reproduced here:



The table shows the solubility of CO2 vs partial pressure. For reference 101 kPa is 1 Atm or sea level pressure. So.. to double the CO2 solubility you have to double pressure. In theory yes you can build a smaller reactor if the CO2 is maintained at high pressure but if you actually calculate how to do that then the pressure requirements are high (> 20psi to be meaningful). Certainly not something you are going to attempt with a plastic setup.

The table also tells us of another important thing - what is the max amount of CO2 water can hold at various temperatures. At 25C and atmospheric pressure the mole frac is .622 which equals 622 ppm.

If CO2 solubility is so high then why is it so hard to get CO2 levels up in our tanks? This is because the solubliity data only tell us about the equilibrium condition and nothing about rates of absorption. For example if you place 100% CO2 gas in contact with water in a enclosed tank and come back after 1 week the CO2 concentration in water will be 622 ppm. Instead if you had tried to measure CO2 ppm within 1 hr, 1 day etc of start the level will be lower.

This is because the process of transfer of CO2 into water is itself VERY slow. The rate of diffusion across the water-air boundary is directly proportional to the surface area and contact time. It is also dependent on the % saturation or gradient of CO2. A 20ppm solution (like in a aquarium) is only (20/622) 3.2% saturated and will take up CO2 almost as fast as a 0ppm solution.

So what does this mean for design principles of Cerges or RG reactors? Well, the two parameters that control CO2 absorption are surface area and contact time. Flow rates of water and CO2 pressure are really not going to make any meaningful difference.

An effective reactor will maximize contact time and surface area. It can be done by bubbles (as in my design) or by a trickle filter design with water trickling over a canister filled with media and CO2 gas. Water flow rate does not have to be that high. If the outflow is at 100 ppm CO2 then its still at <20% saturation and the kinetics of CO2 absorption are not significantly affected.

Next- I will post results of some experiments I conducted with my setup on CO2 absorption rates under different conditions.
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post #15 of 24 (permalink) Old 03-08-2014, 04:56 AM
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Quote:
Originally Posted by rezco View Post
Tom- I agree it can be smaller.This time I am looking just to explore some concepts and gather data to support a smaller design. For a 100-200 gal tank I would probably aim for a 2" dia x 24" tube. Making the venturi and tangential nozzle smaller is bit harder though.

Looking at your design-are you using countercurrent flow - CO2 bubbling up and water flowing down? Any media to break up the bubbles and increase contact time?

Can you share some details on water flow rate, CO2 rate and CO2 concentration in the outflow?

Yes, counter current always, spinning also, so the water spirals down, increasing contact time. The micro bubbles are MUCH hard to dissolve than the large bubbles due to surface tension issues. So any reactor that can avoid them most of the day, is going to run better. The bubbles are good to purge the gas build up however and to stick to plants and pull off epiphytic algae/periphyton etc.

But you do not need this the entire light cycle.

I like the float switch idea.

In a 3 x 18" reactor chamber and 3/4" in/out, you can run about 400gph easily. The venturi is easy to add by drilling a 3/16" hole and adding rigid airline to whatever depth you chose to purge the gas inside the reactor tube, I tend to do about say 4" in an 18" deep tube. This rigid airline is attached to a flex airline tubing etc and runs back to the feed pump for the reactor. Once gas builds up, the gas is then atomized and sent back through the reactor, but some of the gas passes right through and thus purges the reactor chamber of excess gas.

You want 100% effiency and you get it, till the gas bubble builds inside the chamber, but at that point, you need to waste some CO2/reduce the efficiency.
The misting does this effectively, and instead of continuous rise in CO2 ppm throughout the day, we have a nice leveling effect about 2/3rds the way through a photo cycle.
The misting only occurs after the chamber fills up later in the day, it's 100% till then.

This design is far superior than anyone else's design. Because it does this with the CO2:



And does not keep building up higher and higher.
It's self leveling, no other CO2 reactor design I've seen from anyone or any maker does this.
It's not about being 100% efficient the entire time you add CO2.
Now you can do the same sort of thing with a pH controller but my design is cheaper and much more reliable.
It's designed based on the planted tank's needs/ideal conditions, not anything else.
You add another rigid airline say to 16" depth inside the chamber, this is the CO2 input and bubble counter.
It's simple, easy to build and effective. Cerges, and most any design can have it added pretty easily.




Regards,
Tom Barr

Last edited by plantbrain; 03-08-2014 at 05:06 AM. Reason: Cuz
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