Why depth has an effect on sorb capability to scrub CO2

[fsardone]

Check for interesting WOB & scrubber docs at Theory&Docs | DIY Rebreathers

Dear 1c3d1v3r,

Thank you for your post. Thermodynamics 101 is nice but I was not arguing how temperature affects the reaction (although one of my references was talking about the increased cool down due to increased density post #2).

Your post also contained quite an interesting link to the DIY page, which in turn had quite an interesting paper downloaded from DTIC and archived in rubicon foundation but based on research of the department of the Navy.

http://dspace.rubicon-foundation.org/xmlui/bitstream/handle/123456789/4104/ADA160181.pdf?sequence=1

At page 25 of such paper you will find the experimental justification for what happened on Zord which is kindergarten type, but not scientifically unfounded.

Allow me to introduce the table I referenced, a table from Wikipedia (if you do not trust wiki you can look it up yourself in the wiki references) with some physical parameter of gases at play here and a couple of relations to calculate Reynold number in a bead of solid particles.

You will notice how the canister efficiency is related to pressure (the curved lines into the graph) and Reynold number (horizontal axis or abscissa) while the efficiency of the canister is on the vertical axis.

We also have to assess how Re is connected to pressure. Obviously Reynolds number is a fundamental dimensionless quantity that is used to predict on large scale fluids' movements. For more (sorry for the non academic reference but I am privileging ease of access to academic rigor) see:
Reynolds number - Wikipedia in a packed bed (like a scrubber canister) is
Reynolds number is (using the fluid phase and not the fluidized bed equtions):

here rho is the density and mu is dynamic viscosity
(Vs and D I will assume independent from depth since are diameter of the canister and speed of gas)

Now let's see ho mu affect Re and if there is correlation to density hence pressure.
If we assume the kinetic theory of gases is applicable (pressure and temperature non extreme) Viscosity is independent from pressure and increase as temperature increase. Please refer to:
Viscosity - Wikipedia

we also find:

which gives us an ideo on how to calculate Re. I will save you from that and let's assume a very low Re for a scrubber bead at 1 bar Re=20, keeping everything else constant and going to 30 meters (we now have 4 bar) Re=80 because density has tripled. Therefore if we go into the graph with this data we see

We therefore see that efficiency went from approx 0.4 to 0.3 a loss of 25%.
Hope this explanation is less kindergartenly and based on experimental data, logic and fluidodynamics.

All of those other gases: Are not reacting with the sorb. Only the CO2 is. And sure, they might be passing through the sorb, but the efficiency of the sorb is not being diminished until it has been exhausted by ONLY CO2.

Superlyte27,
Your assumptions are your windows on the world. Scrub them off every once in a while, or the light won't come in.
(not mine but Asimov's)
Cheers

 

[Superlyte27]

Dear 1c3d1v3r,

Thank you for your post. Thermodynamics 101 is nice but I was not arguing how temperature affects the reaction (although one of my references was talking about the increased cool down due to increased density post #2).

Your post also contained quite an interesting link to the DIY page, which in turn had quite an interesting paper downloaded from DTIC and archived in rubicon foundation but based on research of the department of the Navy.

http://dspace.rubicon-foundation.org/xmlui/bitstream/handle/123456789/4104/ADA160181.pdf?sequence=1

At page 25 of such paper you will find the experimental justification for what happened on Zord which is kindergarten type, but not scientifically unfounded.

Allow me to introduce the table I referenced, a table from Wikipedia (if you do not trust wiki you can look it up yourself in the wiki references) with some physical parameter of gases at play here and a couple of relations to calculate Reynold number in a bead of solid particles.

View attachment 474991
You will notice how the canister efficiency is related to pressure (the curved lines into the graph) and Reynold number (horizontal axis or abscissa) while the efficiency of the canister is on the vertical axis.

We also have to assess how Re is connected to pressure. Obviously Reynolds number is a fundamental dimensionless quantity that is used to predict on large scale fluids' movements. For more (sorry for the non academic reference but I am privileging ease of access to academic rigor) see:
Reynolds number - Wikipedia in a packed bed (like a scrubber canister) is
Reynolds number is (using the fluid phase and not the fluidized bed equtions):
View attachment 474992 here rho is the density and mu is dynamic viscosity
(Vs and D I will assume independent from depth since are diameter of the canister and speed of gas)

Now let's see ho mu affect Re and if there is correlation to density hence pressure.
If we assume the kinetic theory of gases is applicable (pressure and temperature non extreme) Viscosity is independent from pressure and increase as temperature increase. Please refer to:
Viscosity - Wikipedia
View attachment 474993
we also find:
View attachment 474994
which gives us an ideo on how to calculate Re. I will save you from that and let's assume a very low Re for a scrubber bead at 1 bar Re=20, keeping everything else constant and going to 30 meters (we now have 4 bar) Re=80 because density has tripled. Therefore if we go into the graph with this data we see

View attachment 474995
We therefore see that efficiency went from approx 0.4 to 0.3 a loss of 25%.
Hope this explanation is less kindergartenly and based on experimental data, logic and fluidodynamics.




Superlyte27,
Your assumptions are your windows on the world. Scrub them off every once in a while, or the light won't come in.
(not mine but Asimov's)
Cheers

Just curious, but when is the last time you did a 300’ 10 hour dive on CCR?
I’m pretty happy that Akimbo chimed in. I count him as one of the biggest sources of knowledge when it comes to deep diving, closed circuit info, and saturation stuffs.

You are the bumblebee. Engineers say is shouldn’t be able to fly. But it does.
I’m out. Have a great day everyone.

 

[fsardone]

The major difference is scrubber capacity was designed for at least 24+ hours for four people in deck chambers and gas is circulated by powered blowers. Ambient flow rates through the scrubber varied a little due to gas density, but blowers were in the 1/4+ HP range so depth had very little impact.

Ok, but this is a huge difference because in a rebreather the gas goes through the absorbant bead exactly onle for each breath the diver takes. The flow in a recirculating system can be many times higher than the sum of the sum of tidal volume of all occupants per minute.

Cheers

 

[fsardone]

You are the bumblebee. Engineers say is shouldn’t be able to fly. But it does.
I’m out. Have a great day everyone.

Maybe ...
pretty sure the people at APD and Sherwater are also engineers.
And they say that increased depth negatively affects scrubber efficiency, has it says the Navy in the linked manual.
[edit]
Their conclusionwere based on experimental data collected in testing and not by just sing a mathematical model.
As per the bumblebee cannot fly urban legend plase see:
https://www.snopes.com/fact-check/bumblebees-cant-fly/

The conclusion is:
The real lesson to be gleaned from this myth isn’t that scientists are so blinded by technicalities that they overlook what is painfully obvious to everyone else (namely, that bumblebees really do fly), but that one needs to understand there can be quite a difference between a real-life concept and a mathematical model of it.

And that is why observation and experimental data is used to confirm validity of mathemathical models. Also the reason why constructor warnings should not be discarded.

Cheers
Fabio

 

[1c3d1v3r]

We also have to assess how Re is connected to pressure. Obviously Reynolds number is a fundamental dimensionless quantity that is used to predict on large scale fluids' movements. For more (sorry for the non academic reference but I am privileging ease of access to academic rigor) see:
Reynolds number - Wikipedia in a packed bed (like a scrubber canister) is
Reynolds number is (using the fluid phase and not the fluidized bed equtions):
View attachment 474992 here rho is the density and mu is dynamic viscosity
(Vs and D I will assume independent from depth since are diameter of the canister and speed of gas)

Now let's see ho mu affect Re and if there is correlation to density hence pressure.
If we assume the kinetic theory of gases is applicable (pressure and temperature non extreme) Viscosity is independent from pressure and increase as temperature increase. Please refer to:
Viscosity - Wikipedia
View attachment 474993
we also find:
View attachment 474994
which gives us an ideo on how to calculate Re. I will save you from that and let's assume a very low Re for a scrubber bead at 1 bar Re=20, keeping everything else constant and going to 30 meters (we now have 4 bar) Re=80 because density has tripled. Therefore if we go into the graph with this data we see

View attachment 474995
We therefore see that efficiency went from approx 0.4 to 0.3 a loss of 25%.
Hope this explanation is less kindergartenly and based on experimental data, logic and fluidodynamics.

Thanks for the reference to Reynolds number. I totally forgot about laminar to turbulent flow even though I had read the "Design Guidelines for Carbon Dioxide Scrubbers" document. I still see this totally unrelated to the Zord explanation. Reynolds number is more about real gas velocity in turbulent flow differing from the linear velocity. Not about inert gas blocking chemical reaction.

 

[fsardone]

I still see this totally unrelated to the Zord explanation. Reynolds number is more about real gas velocity in turbulent flow differing from the linear velocity. Not about inert gas blocking chemical reaction.

Every simplification will run the risk of being imprecise and challenged.
The fact that the increased d nsity affects the flow, by increasing the Re and also that cools down the reaction bed, will make the efficiency go down and breakthrough happening well before all the scrubber is exhausted b CO2.

The only purpose of this is to warn people that pushing the scrubber might be feasible at 10m but it might kill them at 50.
I was not trying to publish a paper to pass peer scrutiny

Cheers

 

[Superlyte27]

I was not trying to publish a paper to pass peer scrutiny

Cheers

Let me know when you do some real dives.

 

[fsardone]

Let me know when you do some real dives.

Why not,
Fabio Sardone videos - Dailymotion

Cheers

 

[Superlyte27]

Why not,
Fabio Sardone videos - Dailymotion

Cheers

So no big dives then?
I only watched a couple of videos. But I’m guessing if you think your rebreather is limited by depth, the dives running about 3-4 hours are pretty standard? Is there a specific video I should look at that might show a 6-10 hour dive?

 

[fsardone]

So no big dives then?
I only watched a couple of videos. But I’m guessing if you think your rebreather is limited by depth, the dives running about 3-4 hours are pretty standard? Is there a specific video I should look at that might show a 6-10 hour dive?

My renreather has a 180 minutes scrubber.
Of which 100can be below 50m.
One thing flying has tought me, is to stay within the limits of the equipment that keeps you alive.
I start to see this as a personal unwarranted attack from somebody that till now has shown no use of logic to refute my arguments.

Did I do something wrong to you? Or are you afraid to be wrong?
Anyway this is going offtopic and is against scubaboard policy.
I shall stop here.

Cheers