Physchrometry formula for temperature above 200DegoC

[francoisp]

Hello,

I am trying to write a code to evaluate the mixing ratio, dew point temperature as a function of dry and wet bulb temperature.
There are lots of correlation out there and the ASHRAE standard gives some useful hints and formulae.
For the evaluation of water vapour pressure which is key to the computation of dew point, I have looked at using either Sonntag, Hyland and Wexler formulae, however, the accuracy above 200DegoC decrease rapidly and the error can be up to 15%.
SO, Has anybody done similar calculation for lower and higher temperature and would like to share some of his / her knowledge.

Much appreciated.

Francois

 

[IRstuff]

Does psychrometry even make sense at 200ºC?

TTFN

FAQ731-376

 

[zdas04]

Good point IRstuff, at that temperature you would have to use steam tables (probably the superheated steam tables since the existence of a coherent gas/liquid interface is unlikely unless pressure is really high) to get any useful information.

David

 

[francoisp]

Thanks for your comments,
OK, Partially agree.
However with reference to Psychrometric Tables and Charts from Zimmerman and Lavine (Ed 1964), the table goes much further than 200oC.
For instance at 500oF (~277oC) with a mixing ratio of 0.6kg/kg the dew point would be about 84DegC (at Pressure 101253 Pa), and that is reading from Pyschrometric charts.
People usually know the 'Normal Psychrometric Chart' which usually goes to 35oC.

I understand that these table have been computed somehow, and that's what I am interested in.
Any idea where those are coming from ?

Francois

 

[IRstuff]

I would question whether "accuracy" has any meaning in this context. My original comment was to the effect that wet bulb temperature and dew point would be of purely academic interest, since wet bulb temperature probably cannot be accurately determined at 277ºC and a dew point of 84ºC is pretty much likewise, since steam tables could definitely give you that sort of information more accurately.

Just because you can do the calculation, doesn't necessarily make it worth the expenditure of resources to do it.

TTFN

FAQ731-376

 

[hacksaw]

it is a real problem and easily solved if you have the equations of state. most chemical eng. hb cover it, as well as first year texts.

 

[francoisp]

Thanks Hacksaw !

Unfortunately haven't got many HB of chemical engineering.
found this link though:

http://www.mrc-eng.com/Downloads/Moist Air Props English.pdf

Looks interesting. They also seem to be using the Hyland and Wexler factor.
Their equations for estimating the dew point looks a bit difficult to digest. If you have any links or book ref I would be interested.

Thanks

Francois