Coolprop Calculated Enthalpy of Air does not match a tabulated value.

[Chris Pounds]

Hi, 

Im using Coolprop (Python module) for the first time to do gas turbine analysis modelling the gasses as ideal gasses.

I have a problem with understanding the outputs I get from Coolprop and comparison to tabulated values I find either in textbooks or online. 

With T = 500k, P = 101325 Pa; 

CoolProp.CoolProp.PropsSI('H','T',500,'P',101325, 'Air')

returns an enthalpy at 500K and 1 atm of 629395 J/Kg or 629.35kJ/kg. 

The tables I have available to me say that the enthalpy of air at these conditions should be 503kJ/kg. 629.35kj/kg is in the region of ~620K. 

Plainly I am missing something, but I dont know what it is. I suspect theres an additional step I need to do to convert the value I get from CoolProp to make it line up.

Thanks,

Chris Pounds.

[Matthis Thorade]

Hi,

enthalpy, entropy and internal energy are relative properties, just a difference to some arbitrary reference point.

Are other properties like cp, density, speed of sound all as expected? If yes, then you are just using a different reference point.

• https://github.com/CoolProp/CoolProp/blob/master/FAQ.md
• https://pages.nist.gov/REFPROP-docs/#reference-states-enthalpy-and-entropy-differences

Best regards,

Matthis

[Chris Pounds]

Hi,

Yes, the other properties match pretty well.

I have tried to modify the reference point using CoolProp.CoolProp.set_reference_state function, but trying to set either ASHRAE or IIR doesnt work. An error message is returned saying that theres a runtime error in the QT flash equation and the temperature is out of the limits.

The last option, NBP, isnt it either. 

I guess the next thing to try would be to set it manually via molar values for density, enthalpy and entropy, but from a few minutes experimentation I'm not sure this is the solution either.

I think I can get where I need to be by calculating heat content of a gas via m*Cp*T , but it would be nice to have a one step lookup.

Thanks,

Chris.

[Ian Bell]

Can you show your runtime message with setting reference state?

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[Chris Pounds]

Sure.

[Ian Bell]

Sorry of course that makes sense now to me.  The IIR and ASHRAE reference states are impossible because they are based on saturated liquid at 0C and -40 C and both of those temperatures are above the maximum temperature allowed for liquid vapor phase equilibria for air (your 132.6312 K in the error message).  NBP is your only standard reference state option.  You could also specify the numerical value directly.

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[Chris Pounds]

Ok, so how can I do that without running into the same problem while defining the reference point?

I tried giving CP.set_reference_state() a go with the molar density, enthalpy and entropy of air at 300K , using CoolProp itself to give me those values as in the example on the relevant Coolprop page (http://www.coolprop.org/coolprop/HighLevelAPI.html#reference-states) but there was no difference - either it doesnt work or I just gave it a circular reference point. 

[Ian Bell]

Can you please show your code? 

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