Mixing Two Gases: How to Calculate Combined Volumetric Flow Rate?

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Discussion Overview

The discussion revolves around calculating the combined volumetric flow rate of two different gases being mixed, with a focus on whether the flow rates can be simply added together. The context includes considerations of ideal gas behavior, compressibility, and the conditions affecting the gases involved.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant inquires about the method to calculate the exiting volumetric flow rate of mixed gases and questions if simply adding the flow rates is a valid approximation.
  • Another participant suggests that assuming ideality allows for the addition of flow rates, but notes that real mixing may cause a small change in volume.
  • A participant mentions specific gases (N2 and air, with potential inclusion of CO2 or H2) and expresses confusion regarding the compressibility of gases and the validity of adding their flow rates.
  • Concerns are raised about the conditions (temperature and pressure) affecting the gases and whether they can be treated ideally for the purpose of flow rate calculations.
  • One participant discusses the conservation of mass and questions how the assumption of equal densities among the gases would hold, given the complexities of ideal gas behavior and varying conditions.

Areas of Agreement / Disagreement

Participants express uncertainty about the validity of approximating the combined flow rate by simple addition. There is no consensus on whether the gases can be treated as ideal under the given conditions, and multiple viewpoints regarding the impact of compressibility and density differences are present.

Contextual Notes

Participants highlight the importance of considering temperature and pressure conditions, as well as the potential effects of compressibility, which may complicate the assumption of additive flow rates.

Who May Find This Useful

This discussion may be useful for individuals working on gas flow systems, particularly in contexts involving mixing gases and flow rate calculations in engineering or experimental settings.

Red_CCF
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Hi

I'm working on a project where I have a two flow controller controlling the volumetric flow rate (in the order of 0.01-0.1ml/min) of two different gases. The gases are mixed together and I'm wondering if there's a way to figure out the exiting volumetric flow rate of the combined flow and what kind of information is required? Is just adding the two volumetric flow rate (as if the gases are a liquid) a good approximation?

Thanks
 
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What are the gases? Assuming ideality, then yes, that's exactly what you would want to do.

However, in reality, there might be a small change in volume from mixing.
 
pa5tabear said:
What are the gases? Assuming ideality, then yes, that's exactly what you would want to do.

However, in reality, there might be a small change in volume from mixing.

I'm doing N2 and air for now but CO2 or H2 may also be involved instead of one of N2 or air. So ideally, one can add the volume flow rates together? I'm a bit confused on why since gases are compressible.

Thanks
 
Red_CCF said:
I'm doing N2 and air for now but CO2 or H2 may also be involved instead of one of N2 or air. So ideally, one can add the volume flow rates together? I'm a bit confused on why since gases are compressible.

Thanks

Okay good point. I wasn't thinking very well. You will have to take into account the conditions at each point (temp, pressure). I was thinking that you could treat them ideally, and if the conditions are the same, then you could expect the volumes to be additive.

I'd expect all those gases mentioned to be near ideal, so using the ideal gas law should be fairly accurate.
 
pa5tabear said:
Okay good point. I wasn't thinking very well. You will have to take into account the conditions at each point (temp, pressure). I was thinking that you could treat them ideally, and if the conditions are the same, then you could expect the volumes to be additive.

I'd expect all those gases mentioned to be near ideal, so using the ideal gas law should be fairly accurate.

I'm still puzzled with this approximation. By conservation of mass:
mdot1 + mdot2 = mdottotal and if the volume flow rates are additive and ignoring compressibility effects (as my flow rates are quite low)

I get ρ1*Vdot1 + ρ2*Vdot2 = ρtot*Vtot and if Vtot = Vdot1+Vdot2 this implies that all the density values are equal? I'm not sure how even assuming that the gases are ideal and have constant temperature and pressure will allow this approximation to be valid?

Any help is appreciated
Thanks
 

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