Acentric factor of air

[jonas-z]

Hello,

how is the acentric factor of air calculated? The output of PropsSI('acentric', 'air') is 0.0816749632704
I wonder because the acentric factors of oxygen and nitrogen are 0.0222 and 0.037200000000000004.

[ibell]

Hmm that might be a bug. It is based on the saturation pressure, and I
agree that this value doesn't seem correct to me.
On Aug 10, 2015 8:20 AM, "phileasfoggy" notifications@github.com wrote:

Hello,

how is the acentric factor of air calculated? The output of
PropsSI('acentric', 'air') is 0.0816749632704
I wonder because the acentric factors of oxygen and nitrogen are 0.0222
and 0.037200000000000004.

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#772.

[jonas-z]

OK, thanks so far.
What about the Phase? CoolProp.CoolProp.PhaseSI('P',101325,'T',300,'air') returns
'supercritical_gas' shouldn't be 'gas' the correct Phase?

[ibell]

According to our definition of phase, gas is only for temperatures below
the critical temperature. See for reference
http://www.coolprop.org/coolprop/HighLevelAPI.html#phasesi-function
On Aug 10, 2015 8:45 AM, "phileasfoggy" notifications@github.com wrote:

OK, thanks so far.
What about the Phase? CoolProp.CoolProp.PhaseSI('P',101325,'T',300,'air')
returns
'supercritical_gas' shouldn't be 'gas' the correct Phase?

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#772 (comment).

[ibell]

It's a bug, REFPROP says it should be 0.0335.

[ibell]

Actually, I am not so sure about this after all. Air is a mixture, and the acentric factor is only defined for pure fluids.

[ibell]

Perhaps we should remove it entirely.

[jonas-z]

Maybe. I used CoolProp to get the Density of air. I compared the values to the results of cubic real-gas-equations. To use the cubic real-gas-equations I had to define critical properties and an acentric factor.
I calculated the values by using the mole fraction Xi.

[ibell]

Ok, the "fix" for this problem is going to be to remove the acentric factor and make it equal to infinity. I couldn't come to an agreement about what the acentric factor for air should be, so the safest answer is: not defined.