ideal gas heat capacity

[Richard Aust]

Hi everybody,

I am using CoolProp 6.4.1 with MatLab and I have some problems with the computation of the ideal gas heat capacity:

KM = 'R134a';
T0 = 273.15;
p_1 = 12e5;
T_1 = 55.1 + T0;
pcrit = py.CoolProp.CoolProp.PropsSI('PCRIT', KM)
Tcrit = py.CoolProp.CoolProp.PropsSI('TCRIT', KM)
cp_1 = py.CoolProp.CoolProp.PropsSI('CP0MASS', 'P', p_1, 'T', T_1, KM) / 1000
cv_1 = py.CoolProp.CoolProp.PropsSI('CVMASS', 'P', p_1, 'T', T_1, KM) / 1000

p_2 = 14e5;
Ts1 = py.CoolProp.CoolProp.PropsSI('T', 'P', p_1, 'Q', 0, KM)
Ts2 = py.CoolProp.CoolProp.PropsSI('T', 'P', p_2, 'Q', 0, KM)
cp_2 = py.CoolProp.CoolProp.PropsSI('CP0MASS', 'P', p_2, 'T', T_1 + T0, KM) / 1000
cv_2 = py.CoolProp.CoolProp.PropsSI('CVMASS', 'P', p_2, 'T', T_1 + T0, KM) / 1000

I made shure that I am below the critical point and within the gaseous phase - I get:

cp_1 = 0.8860

cv_1 = 0.8931

cp_2 = 1.3017

cv_2 = 1.2250

Now - cp and cv should not depend on pressure for an ideal gas and besides cp_1 < cv_1 appears to be rather unusual. Anybody out there with an idea what's wrong ...?  

[Ian Bell]

Two issues here: 

In your calculation cp_2 uses a different temperature than cp_1, so no surprise that the cp0 is different

You are mixing ideal gas and real gas specific heats.  CV0MASS is not a supported output, but if it were, the difference between CV0 and CP0 would be a factor of the gas constant.

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[Crístofer Hood Marques]

Hello,

I am not sure if this is the problem, but you are mixing an ideal property (CP0MASS) with a real property (CVMASS). I believe you should obtain the CV0MASS by:

gas_constant/molar_mass = CP0MASS-CV0MASS

Kind regards.