Reactor length - The Planted Tank Forum
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post #1 of 16 (permalink) Old 01-06-2006, 03:46 AM Thread Starter
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Reactor length

Assume a reactor is made from 2" PVC and is mounted vertically, with flow entering the top and exiting at the bottom. If it is not filled with bio balls or other media, does length have any impact on water flow? Meaning, if you have the room should you make it as long as possible, or does the pump have a hard time pushing the extra water volume at some point?
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post #2 of 16 (permalink) Old 01-08-2006, 01:08 PM
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I imagine the length and volume would effect it at some point, but exactly "where" that point is... you got me. It would depend on pump/filter size too. I imagine some of the mathemeticians here could calculate volume vs gph and get close, but why bother.
If you are doing an extra large tank then you may need to add a second reator, if the tank is 125G or so then you would not need to make it extra long to begin with.
How big a tank you talking ?
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post #3 of 16 (permalink) Old 01-09-2006, 03:19 PM Thread Starter
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Buck, the tank is 150g. I was asking about longer reactors because I thought it would make plumbing and mounting easier in my application. When this thread didn't draw any responses I decided to just go with a 24" reactor.

BTW - do you have any ideas regarding my other post about PVC types?
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post #4 of 16 (permalink) Old 01-09-2006, 09:25 PM
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Zelmo,
I currently run a 150g tank with a single 2" diamater ~24" long Rex Grigg style reactor. It works just fine. I tried a 1.5" diamater 24" long reactor and found that it did not dissolve all the CO2. My reactor is also tilted by about 30 degrees.

So my advice would start with a 2" dia 24" long reactor and experiment from there. Every setup is unique, and you will have to figure out what works best for you. Good luck!

Robert
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post #5 of 16 (permalink) Old 01-09-2006, 09:28 PM Thread Starter
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Thanks, Robert. When you say it is tilted 30 degrees, is that with the CO2 connection on the underside? Did you try it before you tilted it?
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post #6 of 16 (permalink) Old 01-09-2006, 09:51 PM
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Zelmo,

The field of fluid dynamics is quite interesting and was my favorite class in college. Flow through a pipe is actually quite difficult to model. Near the walls of the pipe the velocity of the water is almost zero while the velocity in the center is quite fast. What adds to the confusion is that near the walls, where flow is slow, laminar flow prevails. Out in the middle of the tube/pipe/river/ditch the flow is turbulent, which helps to churn up CO2 bubbles, but which also causes more energy loss from friction.

Since the cross-sectional area of the reactor is actually enormous compared to the 5/8" or 1/2" tubing, the velocity through the reactor itself is small, allowing the CO2 bubbles to rise up against the flow until they are quite miniscule. This low velocity also means that resistance to flow is very, very small in the reactor portion of the assembly. In English - you'll get more resistance from 4 inches of tubing than you will from 24 inches of reactor. Nobody worries about an extra 4 inches of tubing. Much more important than the length of the reactor is the transition between tubing and the large pipe. If you have a long, straight, gradual transition there will be MUCH less turbulence & resistance than if you have a short, abrupt, transition with a 90 degree turn in it.

That said, the length isn't all that important once you get to a certain point. Once the bubbles get small enough, they'll move right along with the flow no matter how long the pipe is. Tilting the whole thing a few degrees allows the bubbles to slide out of the high velocity center & climb up along the wall where the counter-flow is slower. This prolongs exposure time and improves the overall efficiency.

It's kinda fun when hobby & science collide - makes you feel like Einstein when something actually works.
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post #7 of 16 (permalink) Old 01-09-2006, 10:11 PM
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Quote:
Originally Posted by zelmo
Thanks, Robert. When you say it is tilted 30 degrees, is that with the CO2 connection on the underside? Did you try it before you tilted it?
I did try it straight up and down first, and found that there were still some bubbles coming out the bottom end and into the tank.

The CO2 connection is at the top.

Here's a cpl of pics:

in operation:
http://static.flickr.com/15/69211682...g?v=1133489831
The reactor is cable tied behind the two pentair aquatic modules.
The blue line is from the CO2 tank into the reactor. You can't really see in this picture how it goes in.

My first attempt with 1.5" diamater PVC, the 2" one looks exactly the same except the tee at the bottom is oriented differently:

http://static.flickr.com/33/68158710...g?v=1133235694


Once again it's a Rex Grigg style reactor (do a search through these forums to find the thread, I believe it's in the DIY section). It's super easy to build, it took me maybe 10 minutes to assemble, most of the time was spent at lowes finding all the right parts.

Robert
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post #8 of 16 (permalink) Old 01-09-2006, 10:17 PM Thread Starter
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Quote:
Originally Posted by guaiac_boy
Much more important than the length of the reactor is the transition between tubing and the large pipe. If you have a long, straight, gradual transition there will be MUCH less turbulence & resistance than if you have a short, abrupt, transition with a 90 degree turn in it.
OK, does that mean we want the turbulance or not?

Quote:
Originally Posted by guaiac_boy
Tilting the whole thing a few degrees allows the bubbles to slide out of the high velocity center & climb up along the wall where the counter-flow is slower. This prolongs exposure time and improves the overall efficiency.
Wow, I thought the reason was the opposite - so that you got the bubbles into the middle of the pipe. Thanks for the help.
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post #9 of 16 (permalink) Old 01-10-2006, 12:05 AM Thread Starter
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Quote:
Originally Posted by wob
Once again it's a Rex Grigg style reactor (do a search through these forums to find the thread, I believe it's in the DIY section). It's super easy to build, it took me maybe 10 minutes to assemble, most of the time was spent at lowes finding all the right parts.

Robert
I have reread the Rex Reactor thread many times.

Well, I think I just spent more time trying to figure out how this works and the characteristics of PVC (see other post) than even the time at Lowes, HD, and another local hardware store.

Thanks for the pics. Those along with guaiac_boy's great description of what goes on inside gives me the info I needed.
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post #10 of 16 (permalink) Old 01-10-2006, 01:00 AM
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Quote:
Originally Posted by zelmo
I have reread the Rex Reactor thread many times.

Well, I think I just spent more time trying to figure out how this works and the characteristics of PVC (see other post) than even the time at Lowes, HD, and another local hardware store.

Thanks for the pics. Those along with guaiac_boy's great description of what goes on inside gives me the info I needed.
No worries, it took me a solid month of constant reading here and other places about how to do pressurized CO2 with a DIY reactor, so you're not alone

Robert
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post #11 of 16 (permalink) Old 01-10-2006, 02:25 AM
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Quote:
Originally Posted by zelmo
OK, does that mean we want the turbulance or not?
Turbulence = wasted energy = more resistance = less flow from the filter or pump. The only theoretical advantage would be slightly more mixing of the CO2, but the reactor design should be plenty good without this.
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post #12 of 16 (permalink) Old 01-12-2006, 01:01 AM Thread Starter
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Robert -

In the picture of your reactor that is working, although you can't see the CO2 connection, it seems it is on the same side as the output at the bottom and therefore tilted up. I didn't think about this the first time I looked at it, but doesn't that cause a CO2 bubble to build up inside the T where the connection is made? Is that part of the plan?

BTW - what pump are you using and how is it working on your 150?

Last edited by zelmo; 01-12-2006 at 03:05 PM.
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post #13 of 16 (permalink) Old 01-12-2006, 09:18 PM
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Quote:
Originally Posted by guaiac_boy
Zelmo,

The field of fluid dynamics is quite interesting and was my favorite class in college. Flow through a pipe is actually quite difficult to model. Near the walls of the pipe the velocity of the water is almost zero while the velocity in the center is quite fast. What adds to the confusion is that near the walls, where flow is slow, laminar flow prevails. Out in the middle of the tube/pipe/river/ditch the flow is turbulent, which helps to churn up CO2 bubbles, but which also causes more energy loss from friction.

Since the cross-sectional area of the reactor is actually enormous compared to the 5/8" or 1/2" tubing, the velocity through the reactor itself is small, allowing the CO2 bubbles to rise up against the flow until they are quite miniscule. This low velocity also means that resistance to flow is very, very small in the reactor portion of the assembly. In English - you'll get more resistance from 4 inches of tubing than you will from 24 inches of reactor. Nobody worries about an extra 4 inches of tubing. Much more important than the length of the reactor is the transition between tubing and the large pipe. If you have a long, straight, gradual transition there will be MUCH less turbulence & resistance than if you have a short, abrupt, transition with a 90 degree turn in it.

That said, the length isn't all that important once you get to a certain point. Once the bubbles get small enough, they'll move right along with the flow no matter how long the pipe is. Tilting the whole thing a few degrees allows the bubbles to slide out of the high velocity center & climb up along the wall where the counter-flow is slower. This prolongs exposure time and improves the overall efficiency.

It's kinda fun when hobby & science collide - makes you feel like Einstein when something actually works.

Very nicely explained (even in scientific terms)...I noticed this effect you describe so I've always had my reactor tilted to an angle.

Re-boot!
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post #14 of 16 (permalink) Old 01-12-2006, 09:42 PM Thread Starter
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Ed -

I saw in another post of yours where you said changing your filter to one with more flow allowed you to reduce the bubble count. Any idea why?
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post #15 of 16 (permalink) Old 01-12-2006, 11:15 PM
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Quote:
Originally Posted by zelmo
Robert -

In the picture of your reactor that is working, although you can't see the CO2 connection, it seems it is on the same side as the output at the bottom and therefore tilted up. I didn't think about this the first time I looked at it, but doesn't that cause a CO2 bubble to build up inside the T where the connection is made? Is that part of the plan?

BTW - what pump are you using and how is it working on your 150?
You are correct. After the CO2 has been running for a while (around 10hrs) I can hear some bubbles gurgling in that area. I don't think it's a problem though, my CO2 levels are easily over 30ppm (sitting around 40 last time I tested) so I've never really worried about it. By the time morning comes around and the solenoid turns the CO2 back on, the bubbles are gone. And no, it was not part of the plan really, it just kinda worked out to be hooked up that way. I originally was running the reactor straight up and down, but as noted in a previous response, I tilted it to get rid of the bubbles coming out of the bottom.

Also note that the CO2 tubing is pulled into the middle of the reactor (the Rex Grigg Reactor thread describes how you pull the CO2 line through the hole you drilled in the plug of the tee) so the actual outlet of the CO2 tubing is right in the middle of the 2" PVC pipe. So I am guessing there's more CO2 buildup at the top of the reactor than at the tee.

I drive my pentair aquatic modules with a QuetOne 3000 pump. It provides enough flow, I would say just the right amount. I'm guessing my tank get's cycled 2-3x an hour with this pump, which looks to be enough. I may buy a larger pump (the 4000) one day, but it's not a priority.

Robert
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