4 replies to this topic

[Steve Hall]

Imagine an indoor space with a slightly permeable wall separating it from the outside (leaking windows, for example). The indoor space has humidity and pressure control, so it is 10 Pa positive relative to the outside, and the dew point is roughly 17C. This makes the partial pressure of the water vapor approximately 2 kPa.

 

A colleague asserts that if the outdoor dew point is 22C, so the partial pressure of the water vapor is approximately 2.6 kPa, that water will penetrate into the indoor space even though the total pressure is higher indoors.

 

Do you agree?

 

 

Attached Files

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[PingPong]

Yes, I agree with your colleague. It is only partial pressure of diffusing component that matters.

 

Note that in your attachment you interchanged the Total Pressure numbers.

[Steve Hall]

Yes, I interchanged the total pressure numbers.

 

Thanks. So is it a diffusive process since air will continuously flow from indoors to outdoors?

[PingPong]

It is a diffusive process, but any air flow has no influence on the direction of H₂O ttransport.

 

If the wall is only permeable for H₂O , but not for N₂ or O₂ . there will still be transport of H₂O from its higher to its lower partial pressure.

 

Strictly speaking it is the fugacity of the H₂O vapor that drives the diffusion until equilibrium between both spaces is achieved. As we can consider atmospheric air inside and outside to be ideal gases, fugacity equals partial pressure for each component i in the gas mixtures.

[Steve Hall]

Great. Thanks!