Thermodynamics – Discrepancies in R134a Property Tables

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I need internal energy and volume value for R134a refrigerant, to get the values went through steam tables/property tables for R134a

For a particular steam table

At temperature 30 degree celsius, Specific internal energy = 246.14 kJ/kg, specific Volume of gas = 0.026622 m^3/kg,

For one more steam table from another resource

At temperature 30 degree celsius, Specific internal energy = 394.48 kJ/kg, specific Volume of gas = 0.02671 m^3/kg,

Which table values must be considered and why there is such a difference?

Answer 1

As @Chet Miller pointed out in his comment, the reason could be different zero reference states for internal energy for the two tables. Notice the last sentence of the notes at the bottom of your first table. It reads "The enthalpy and entropy values of saturated liquid are set to zero at -40 C".

But heres the important point: For all practical purposes it doesn't matter where the zero reference is set and what the specific absolute values are. That's because when we use these tables we are almost always only interested in changes in the values of internal energy, enthalpy, and entropy, not the absolute values. For that purpose either table would be ok since changes in these properties should be approximately the same.

To illustrate, consider the increase in the specific internal energy of the fluid going from 30 C to 40 C for the two tables.

From the first table:

$\Delta u=250.97-246.14=4.83$

From the second table:

$\Delta u=399.46-394.48=4.98$

So you see the change in specific internal energy of the saturated vapor between the same two temperatures is about the same for both tables.

By the way, your first table appears to align with the following table:

https://www.cambridge.org/us/files/2313/6697/5548/Appendix_C.pdf

Note that in this table the zero reference state for the specific enthalpy and specific entropy of saturated liquid is -40 C per the notes in your first table. The zero reference state for the specific internal energy of the saturated liquid falls between -35 C and -40 C.

Hope this helps.