Steam condensation in the presence of air

[sanka]

I've been reading about this topic in a book recently and it states that when steam (water vapor) condenses on a cold surface in the presence of air, the increase in air concentration at the condensation interface causes a reduction in the steam pressure and hence, temperature. I am wondering why this is the case? Why does the overall system pressure not just increase? For example, in a condenser operating with non-condensables (such as air), I understand that the partial pressure of air will increase at the condensation interface due to the increase in concentration there but would that increase in partial pressure not just increase the overall condenser pressure instead of depressing the steam pressure & temperature?

Please feel free to show me the error of my ways.

 

[SteamKing]

Care to share a reference to what you've been reading?

 

[sanka]

Its in a book called "Liquid-Vapor Phase Change Phenomena" by Van P. Carey.

I have also read some papers which show that both the condensation heat transfer coefficient and heat transfer decrease as non-condensable concentration increases. It makes sense that if the steam temperature decreses due to the presence of air (non-condensable gas), then the heat transfer rate must accordingly decrease. However, if the steam temperature decreases, it implies that the thermal resistance to heat flow decreases and the heat transfer coefficient must then be increasing.

Any ideas?