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Ideal Gas Air Tables

Ideal Gas Air Tables

Ideal Gas Air Tables

(OP)
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/...)

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?

RE: Ideal Gas Air Tables

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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RE: Ideal Gas Air Tables

(OP)
@DRWeig

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

-David

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