Ideal Gas Air Tables

[dfaunce]

Entropy values between SI and English units in the Ideal gas tables of air do not seem to be linearly related...is this true? If not,

Sanity Check:
1 (kJ/kg-K) = 0.238845 (Btu/lbm-R)

If the above is true, take a look at the ideal gas tables for air (here's a link to some

http://energy.sdsu.edu/testhome/Test/solve/basics/tables/tablesIG/igAir-Eng.html)

At the top you can toggle between SI and English units.

Take a look at 300K and at 540R (equivalent temperature values)

@ T=300K the standard entropy (s) value is 1.70203 (kJ/kg-K)
@ T=540R the standard entropy (s) value is 0.60078 (Btu/lbm-R)

According to this: 0.60078/1.70203 = 0.35297
so 1 kJ/kg-K = 0.35297 Btu/lbm-R which disagrees with my above statement.

So I checked again with another equivalent temperature values:

@ T=500K, s=2.21952 (kJ/kg-K)
@ T=900R, s=0.72438 (Btu/lbm-R)

0.72438 / 2.21952 = 0.32637
so 1 (kJ/kg-K) = 0.32637 (Btu/lbm-R)

None of the values seem to match...is there not a linear relationship between entropy units?

 

[DRWeig]

In my experience, absolute values of units in tables, charts, and graphs that depend on temperature need to be adjusted for the offset between 0°C and 32°C, in addition to the conversion factor. In other words, the base temperature at which base values of entropy, enthalpy, and other quantities that vary with temperature must be adjusted.

32°F = 491.67°R and 0°C = 273.15°K, but those are both the same absolute temperature.

s(kJ/kg-K) = X * s(Btu/lbm-R) for differential entropy, or change in entropy, where X is a simple conversion factor without regard to the difference in base temperatures between °K and °R

s(kJ/kg-K) = X * (s(Btu/lbm-R) - Y) for absoute entropy, where X is the same conversion factor from above, and Y is representative of the offset due to difference in base temperatures between °K and °R. The formula is not exact, but generally will be close. It is only exactly accurate at the definition point (0°C? - that is the base for enthalpy, but I'm not sure about entropy). It may also apply to only bone-dry conditions, but I haven't researched this.

Here's an example for enthalpy that I gave a while back:

RELATIVE ENTHALPY OR Δh: Btu/lbm * 2.326 = kJ/kg



ABSOLUTE ENTHALPY OR ha: Approximately (Btu/lbm - 7.686) * 2.326 = kJ/kg ==> Note the word "approximately." The 7.686 figure varies a bit over the range of humidities. It's only exactly valid at bone-dry air conditions (0% relative humidity). One paper (attached) says the answer will be ±5 kJ/kg




Good on ya,

Goober Dave

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[dfaunce]

@DRWeig

This is great info! This makes a lot more sense now. Thanks!

-David