Please enable JavaScript.
Coggle requires JavaScript to display documents.
Real Gases: Enthalpy and Entropy (How to use the enthalpy deviation chart…
Real Gases:
Enthalpy and Entropy
Enthalpy and entropy
Four methods:
Equations(substance specific, normally incorporated in computer programs)
Graphs (substance specific)
Tables (substance specific)
Generic equations
Tables
Entropy at 60MPa, 775ºC
Graphs
Advantages
Not that easy to read
Usually large read-off mistakes
Allow direct determination of all other variables in the graph once ANY 2
variables are given
2 examples
Mollier Diagrams (h-s)
Pressure -enthalpy diagrams
Mollier diagram
(usually for steam)
4-6 variables:
Enthalpy (y axis)
Temperature (isotherms)
Entropy (x axis)
Pressure (isobares)
Quality
sp. Volume (isochores)
Pressure-enthalpy graph
(simple example)
Calculate h and s using
generic deviation method
Just like sp. Volume, deviation of enthalpy and entropy can be described as functions of reduced temperature and pressure
Use these equations to calculate Δh and Δs
Enthalpy Deviation Chart
Entropy Deviation Chart
How to use the enthalpy deviation chart (Tables)
Multiply with the critical temperature to get the enthalpy deviation in kJ/kmol’s
Calculate Pr
and Tr
Convert to kJ/kg by dividing by the molar mass
Molar mass (also called molecular weight) is given in table 1 of the handout;
units are g/mol or kg/kmol
Look up Pc
and Tc
subtract ∆ℎ𝑑 from the enthalpy calculated using ideal gas law to get the actual
enthalpy
Look up enthalpy deviation in “ table” 7
Note that the enthalpy is normalised with the critical temperature and that the units are in kJ/kmol·K
How to use the entropy deviation chart
Look up enthalpy deviation in “ table” 7
Note that the units of the entropy are in kJ/kmol·K
Convert kJ/mol·K to kJ/kg·K by dividing by the molar mass
Molar mass (also called molecular weight) is given in table 1 of the handout;
units are g/mol or kg/kmol
Calculate Pr
and T
subtract ∆𝑠𝑑 from the entropy calculated using ideal gas law to get the actual
entropy
Look up Pc
and Tc
Deviation chart vs Mollier diagram
Mollier diagram:
reading errors
possibly large near critical point
Deviation charts:
model (might be) incorrect (for water)
How to use the enthalpy deviation chart (Book)
Note that figure D2 in your book contains the same information as the figure in
page 41 in the tables. However, enthalpy deviation is differently defined:
h/RTc is plotted, instead of h/Tc.
Look up Pc
and Tc
Calculate Pr
and Tr
Look up enthalpy deviation in “ table” 7
Note that the enthalpy is normalised with the critical temperature and that
the units are in kJ/kmol·K
Multiply with the critical temperature to get the enthalpy deviation in kJ/kmol’s
NB!
Convert to kJ/kg by multiplying with the gas constant
subtract ∆ℎ𝑑 from the enthalpy calculated using ideal gas law to get the actual
enthalpy
The same holds for the entropy deviation (D3 vs. p42)
