enthalpy exact calculation

[zdej]

Hello to all!

I have a question regarding enthalpy calculation... I am trying to do some exergy analysis and I need to have exact enthalpy values... which values for cp,air cp,wv and r (h=cp,a*T+w(r+cp,wv*T)) do I need to use to get exact enthalpy values for moist air (compared to ashrae tables)? using suggested approximate values cp,air=1,006 co,wv=1,86 and r=2501 i get slightly different results than values in ashrae table - error is +/- 0,1 but this can result in a significant error in exergy efficiency in some cases (exergy efficiency larger than 1). I ruled out other possible causes for this...

i tried to get polinomal equations for dry and saturated air enthalpy (form ashrae table) but i do not get good results with excel.. because i need to calculate a lot of different cases, calculating moist air enthalpy 'by hand' from ashrae tables would take A LOT of time...

i hope i was cleat what i need to do

thanks!

 

[cloa]

How many cases have you got? and can you attach what you have tried so far.

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[davefitz]

as I recall, there are published by Nasa exact values for the polynomials used to desicribe the equation of state for air- check the COSMIC website at the Univ of Georgia for the NASA programs for thermodynamic funcitons

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[ione]

You could try the calculator below

http://thermodynamik.hs-zigr.de/fpc/index.php

It should be based on the Hyland Wexler formulation of thermodynamic properties of humid air adopted by ASHRAE.

 

[zdej]

thank you for answering. unfortonately i did not find polynomials (cosmic website), but i did manage to get a bit more accurate polynomials on my own. i hope now it will be OK...

 

[siretb]

Hi

I am not sure I understand what your problem is.

There are many good polynomial expressions (and non polynomial) for the Enthalpy (vapor state) for, separately
nitrogen, oxigen, water, and rare gaz if you wish
You do a two step process
1) determine the composition (H2O content, ....)
2) calculate enthalpy from compnents and temperature

If you could provide an exemple, with your calculations, that would help check

 

[zdej]

thank you for replying, however i managed to get good polynomial expressions so this problem is solved