Liquid and vapor saturation pressures are not the same for some fluids

[ChenYuChih]

Hello Mr. Bell,

Recently I began to do some detailed test the functions of CoolProp v5. I found that PropsSI("P","T",373,"Q",0,"R134a") is not equal to PropsSI("P","T",373,"Q",1,"R134a").

I know this condition is near critical area, but this problem gradually starts since T=358K.

I think this might also happen to other azeotropic fluids. Is it possible to be fixed?

Thank you very much!

Best Regards,
ChenYuChih(David Chen)

[ibell]

That is a bug. I haven't the foggiest idea why you see this behavior, but
we will fix it ASAP

In [1]: import CoolProp

In [2]: CoolProp.CoolProp.PropsSI('R134a','Tcrit')
Out[2]: 374.21

In [3]: CoolProp.CoolProp.PropsSI('P','T',373.5,'Q',0,'R134a')
Out[3]: 4000617.6959242234

In [4]: CoolProp.CoolProp.PropsSI('P','T',373.5,'Q',1,'R134a')
Out[4]: 4004895.7657442535

On Fri, Dec 19, 2014 at 9:34 PM, ChenYuChih notifications@github.com
wrote:

Hello Mr. Bell,

Recently I began to do some detailed test the functions of CoolProp v5. I
found that PropsSI("P","T",373,"Q",0,"R134a") is not equal to
PropsSI("P","T",373,"Q",1,"R134a").

I know this condition is near critical area, but this problem gradually
starts since T=358K.

I think this might also happen to other azeotropic fluids. Is it possible
to be fixed?

Thank you very much!

Best Regards,
ChenYuChih(David Chen)

—
Reply to this email directly or view it on GitHub
#364.

[ibell]

Well, I think this might be an artifact of our solver method. For pure fluids, in the iterative solver we get liquid and vapor pressures and stop when they are close enough to each other. Perhaps the best would be just to always return the mean pressure (between liquid and vapor) to remove this question.

That said, R134a is a problematic fluid since its EOS has two critical points, and also uses a different reducing state than the critical state. I'll make the change to make it more clear.

[ChenYuChih]

Is it possible to use the same equation for T-P saturation curve of dew&bubble point on azeotropic fluids?

[ibell]

No, you have to use the Maxwell criteria of equal pressure and equal Gibbs
function and iterate to get densities of liquid and vapor. For
pseudo-pure, the situation is different.
On Dec 23, 2014 2:57 AM, "ChenYuChih" notifications@github.com wrote:

Is it possible to use the same equation for T-P saturation curve of
dew&bubble point on azeotropic fluids?

—
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#364 (comment).

[ibell]

And now:

In [1]: import CoolProp

In [2]: CoolProp.CoolProp.PropsSI('R134a','Tcrit')
Out[2]: 374.21

In [3]: CoolProp.CoolProp.PropsSI('P','T',373.5,'Q',0,'R134a')
Out[3]: 4002756.730834238

In [4]: CoolProp.CoolProp.PropsSI('P','T',373.5,'Q',1,'R134a')
Out[4]: 4002756.730834238

[ChenYuChih]

Thanks for your help. :)

Is this fix for all pure fluids or just only for R134a?

[ibell]

All pure fluids. Away from the critical point, difference should be <
0.001 %
On Dec 24, 2014 6:45 PM, "ChenYuChih" notifications@github.com wrote:

Thanks for your help. :)

Is this fix for all pure fluids or just only for R134a?

—
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#364 (comment).

[ChenYuChih]

Got it, Thanks!