Optimal amount of CO2 to purge air in a device

[Lyd_h]

<Moderator's note: Moved from a homework forum.>

Dear team, I wish to use CO2 to de-air a device by exploiting the physical properties of CO2 as a heavier, more dense gas to air and therefore displace air in a very thin tube that has a nozzle at the end where both air and CO2 can escape.
I wish to know under what conditions should I study CO2 to effectively remove air. I think that I would need a high flow rate to ensure that diffusion with air within the tube is counteracted, a low velocity of CO2 to ensure laminar flow within the tube to reduce turbulence of CO2 and again ensure optimal air displacement, and deliver the CO2 at a high pressure and high pressure leads to low velocity.
The CO2 would be delivered from a standard cylinder tank that has a pressure gauge attached to it. A CO2 flow meter is attached to the pressure gauge and from the flow meter, 1/4inch tubing of length 120cm is then attached to the device which itself is about 120cm long and has an internal diameter of 8mm.
As there may be a pressure drop along the length of the CO2 tube and device tube, I am not sure if I need to also take this into account.
I thought about having a flow meter at the end of the device where CO2 comes out to look at what flow rate is actually coming out of the device.

2. Homework Equations
Ficks law
Grahams Law
Bernoulli's equation

3. The Attempt at a Solution
Have yet to test out the above assumptions.

 

[BvU]

Hello Lyd, $\qquad$ $\qquad$ !

I thought about having a flow meter at the end of the device where CO2 comes out to look at what flow rate is actually coming out of the device.

Is there any reason the two flowmeters would show something different ?

CO₂ as a heavier, more dense gas

So is the tube vertical ?

effectively

'Effectively' meaning what, precisely ?

As you can see: you ask one question, get three in return

Funny you call 8 mm 'very thin' but don't say anything about the 1/4 "

I think that I would need a high flow rate to ensure that diffusion with air within the tube is counteracted, a low velocity of CO2 to ensure laminar flow within the tube to reduce turbulence of CO2 and again ensure optimal air displacement, and deliver the CO2 at a high pressure and high pressure leads to low velocity

That would count as 2 to 1 in a democratic process, and the 'highs' would carry it

Aren't you making this more diffficult than necessary ?

 

[jrmichler]

You can get some ideas by looking at wire feed welders. They use CO2 or Argon/CO2 as shielding gas, and are up against the same questions of laminar vs turbulent flow to displace air. Use search term wire feed welder gas flow rate to learn about flow rates.

The shielding gas displaces air to improve weld quality. To find if the flow is laminar or turbulent, measure the diameter of a welding nozzle, estimate a range of flow rates from your search results, and calculate some Reynolds numbers. Most any university engineering department will have a machine shop with a wire feed welder, if not, go to the maintenance department. And, like @BvU says, try to avoid overthinking the problem.

 

[Lyd_h]

You can get some ideas by looking at wire feed welders. They use CO2 or Argon/CO2 as shielding gas, and are up against the same questions of laminar vs turbulent flow to displace air. Use search term wire feed welder gas flow rate to learn about flow rates.

The shielding gas displaces air to improve weld quality. To find if the flow is laminar or turbulent, measure the diameter of a welding nozzle, estimate a range of flow rates from your search results, and calculate some Reynolds numbers. Most any university engineering department will have a machine shop with a wire feed welder, if not, go to the maintenance department. And, like @BvU says, try to avoid overthinking the problem.

Hello Lyd, $\qquad$ $\qquad$ !
Is there any reason the two flowmeters would show something different ?
No. But wouldn’t the CO2 delivered flow at the beginning of the tube change at the end of the tube where the nozzle is, given that the tube has wires inside and a PTFE cylinder that is crushed down. So I’d expect there to be resistance to flow from these things? So even though my flow meter is set to deliver say 2l/min, what actually end up being delivered may be less? So is the tube vertical ?
Yes the tube is elevated to allow air to be purged

'Effectively' meaning what, precisely ?
If I were to measure the o2 (as a surrogate of air) and CO2 concentrations, i’d find CO2 in high % and 02 in low % for a given time

As you can see: you ask one question, get three in return

Funny you call 8 mm 'very thin' but don't say anything about the 1/4 "

That would count as 2 to 1 in a democratic process, and the 'highs' would carry it

Aren't you making this more diffficult than necessary ?

Maybe? But to effectively de-air a gas with another gas, aren’t the flow rates, pressure and velocity of the displacing gas important so that you can be sure displacemt of the unwanted air is efficient? N

Hello Lyd, $\qquad$ $\qquad$ !
Is there any reason the two flowmeters would show something different ?
So is the tube vertical ?

'Effectively' meaning what, precisely ?

As you can see: you ask one question, get three in return

Funny you call 8 mm 'very thin' but don't say anything about the 1/4 "

That would count as 2 to 1 in a democratic process, and the 'highs' would carry it

Aren't you making this more diffficult than necessary ?

 

[BvU]

Is there something in my questions that is unclear ? You did not answer any

 

[Lyd_h]

So sorry
I’m still getting used to this forum.
I answered below your questions

Will sneer here as well.

1. The flow meteres are the same, however, the tube has wires and a PTFE cylinder that is crushed down. All of this within the tube I would think would create resistance to flow, so even if I deliver CO2 say at 2l/min from the cylinder, wouldn’t there be loss of this down-stream because the gas would encounter these resistances to flow? So the flow actually coming out of the tube may not be what is delivered?

2. The tube is elevated to allow the de-airing

3. Effective means that as time goes on I would expect that there would be near 100% CO2 and very little 02 (as a surrogate marker of co2) . I would measure this with a gas sensor.

I may be making this more difficult... but, as we’ll be de-airing several devices, I would need the same flow-rate, velocity and pressure of CO2 that would de-air the device. And each of these parameters would have an ideal level?

Thank you!

 

[BvU]

Will sneer here as well.

Fair enough, all in good spirits

1. Flowmeters usually measure flow in Nl/min (or SCFM in some backcountries), i.e. proportional to the mass flow. No leakages --> same mass flow.

2. 'The tube', or 'one end of the tube'? I'll assume the latter

3. If you want less than 1 ppb of O₂ you'll have to flush longer than if you need less than 1 ppm, that's what I meant. 'Near 100%' conveys no information.

has wires and a PTFE cylinder that is crushed down

wires? it it hairy? I assume the crushed PTFE leaves at least some room for the gas to pass ? How much ?

All of this within the tube

Nice to know. Means longer flushing, as you indicated.

You could assume a worst case and treat the volume before the PTFE constriction as an ideally stirred tank

 

[Lyd_h]

Thank you! That’s all really helpful!

Yes my plan was to assume worst case ‘amount of air ‘ within the PTFE tube when the PTFE is a cylinder rather than when it is crushed down because it will be impossible to know the exact volume of dead space/air when it is crushed down.
When it is crushed down it also has a wire running through it which will also take up space within the PTFE cylinder but again it will be tricky to know what volume that small segment of wire is to then subtract that from the total volume of the PTFE cylinder.

So, using are x length of the PTFE cylinder, I will
Estimate the volume of air that needs replacing with co2. When the PTFE cylinder is crushed down it looks like spokes of a wheel type pattern in cross section so of course there’ll be air within the main lumen
and then air in between all the folds.

I know CO2 can get into
All
Of these spaces because I can feel a jet of CO2 out of the other end.

But there will be an optimum flow rate of CO2 needed to fill up the crushed cylinder and displace air, and so I wanted to know the best way to deliver CO2 to reduce turbulence of CO2 with air within the cylinder (which I think needs a low velocity) and I wasn’t sure how pressure of CO2 on the pressure reg fitted into this... Sorry I have probably not made my questions clear from the beginning!

 

[BvU]

Sorry I have probably not made my questions clear from the beginning!

I tend to agree . On the other hand, I commend you for thinking about what exactly is the question to be answered, before working on answering just any question that pops up.

Something I saw makes me raise an eyebrow:

I would measure this with a gas sensor

So why bother to calculate at all ? Flush until O₂ < desired maximum !

 

[LURCH]

Is the air being displaced to prevent corrosion of the equipment, prevent fire, chemically infuse the material, prevent oxidation, or what?

 

[Chestermiller]

This can all be modeled pretty easily. Is the flow rate of 2 l/min roughly what you would plan to use, give or take?

 

[berkeman]

Thread closed temporarily for Moderation...

 

[berkeman]

Thread will remain closed at OP request. Thank you all for some great and helpful replies.