Calculation of dewpoints: water/noncondensible gas mixtures 1

[LaSalle1940]

We all know of psychrometric data that document the thermodynamics of mixtures of water vapor with the all-important noncondensible gas, air. But what about water vapor and other noncondensibles--say, for example, hydrogen? How does one go about estimating dewpoints for such mixtures?

Let's say we have a water vapor/hydrogen mix of known composition at about barometric pressure and a temperature in the ambient range. My first impression is that the dewpoint is a function of the partial pressure of water vapor in the gaseous mix and of the vapor pressure of water at the temperature in question--and therefore, the nature of the noncondensible is irrelevant.

However, that doesn't quite sit right with me: it's my impression that the nature of the noncondensible should influence the dewpoint somehow.

The standard physical properties computation references (e.g., The Properties of Gases and Liquids) address the issue of dewpoints of mixtures of condensibles, but not the type of mixture I'm involved with. I'm looking for a calculation/estimation method, especially with a reference. All help is appreciated. Thanks.

 

[Montemayor]

LaSalle1940:

Gases at relatively low pressures and saturated with water can be calculated for their moisture content using the classic Dalton's Law:

Total System Pressure = sum of the components' partial pressures

Of course, all pressures are absolute. The partial pressure of water in a 100% saturated mixture is equal to its vapor pressure at the system temperature.



Art Montemayor
Spring, TX

 

[hacksaw]

The law of partial pressures works fine here. Montemayor has it right.

 

[LaSalle1940]

That's the conclusion I reached from my first impression of the situation; i.e., that the nature of the noncondensible gas is irrelevant. However, you know how it is: it's all too easy to second-guess oneself and decide you've missed something and the first impression is too simplistic.

Thanks.