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Water Entropy Estimation

entropy and enthalpy of water

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#1 ahmedi

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Posted 19 December 2021 - 04:53 AM

Hi, 

 

I have a question. The standard molar entropy of subcooled/compressed water (25oC, 1 ATM) is given as 70 J/mol. K. When it is converted to J/gm K the value comes about 3.88 J/kg K. But when we look at the steam tables the value of saturated liquid water entropy is given (25 C, 3.6 kPa) as 0.367 J/kg. K. I could not figure out why there is so much of a difference? As we know, entropy is not a strong function of pressure at a given temperature. Could someone kindly throw light on it please?

 

S_o (25 C, 101.32 kPa) = 3.88 kJ/kg.K

S_sat (25 C, 3.6 kPa) = 0.367 kJ/kg K  

 



#2 latexman

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Posted 19 December 2021 - 07:02 AM

Are these two values from two different steam tables?  They may use different reference conditions.



#3 Said Salim

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Posted 20 December 2021 - 02:28 PM

You may check the reference temperatures in both sources.As far as I know the triple point is the reference in IAPWS-97.
You check what is the reference temperature when calculating specific standard entropy to get the value of 3.88 kJ/kg.K

Edited by Said Salim, 20 December 2021 - 02:31 PM.


#4 breizh

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Posted 20 December 2021 - 11:52 PM

Hi,

Consider this link to support your work (calculators )

https://www.spiraxsa...nd-design-tools

Breizh 






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