6 replies to this topic

[dhuber]

Hello all.  I have customers asking if they can use Nitrogen instead of Heluim in a compressed gas emergency inflation system due to availability and cost.  The Helium is currently compressed to ~4200 psig (290 Bar) at 21 degrees C in the system reservior.  The question I need to answer is do they need a larger reservior and/or a higher pressure rated reservior to have the same inflation volume of gas when the system is activated.  The pressure in the system bladders after activiation is ~2.5 psig (1.19 Bar) at 21 degrees c.

 

I have used Ankur's spreadsheet for calculating the z factor for these two gases.  I find little difference in the z factor values when I include the quantum gas correction factors in Tc and Pc for Helium.  I would have expected a significant difference (10-20%) in the required pre-activiation volume/pressure given the differences in the gas properties.  My calculations currently yielding about a 1% difference.  Is this a reasonable assumption?

 

Thank you for any feedback on this topic. 

[PaoloPemi]

you can compare the ratio of densities at the two conditions of pressure with t = 21C

these have been calculated with PR

 

for Helium

p1=1.19 bar.a -> rho1 = 0.194 Kg/m3

p2=290 bar.a -> rho2 = 42 Kg/m3

rho2/rho1 = 216

 

for Nitrogen

p1=1.19 bar.a ->rho1 =1.36 Kg/m3

p2=290 bar.a -> rho2 = 298 Kg/m3

rho2/rho1 = 219

[dhuber]

THank you.  THat seems to confirm my findings.

[dhuber]

After looking into this further, and calculating the mass of the Nitrogen that would be in the system, I find this to be only about 1/2 of what I would expect it to be.  The Helium looks to be correct.  The initial volume in the high pressure cylinder is .0181 m³  The final volume is 4.106 m³

[dhuber]

After again looking at this I found my error regarding the Nitrogen mass.  Your original calculation is correct.

Thank you

[dhuber]

_{My real question is regarding the z-factor calculation.  Is it appropriate to use the Newton correction factors for Pc and Tc  with quantum gases (in this case Helium)?  It makes a significant difference in the resultant compressibility factor.}

[PaoloPemi]

the values shown in my previous post have been calculated in Excel with the Peng Robinson package available in Prode Properties library, you may download a copy from prode.com if you wish to test.

For density or volume you can obtain values a bit more accurate with models as BWR which have specific parameters,

but I do not expect large differences,

I have also REFPROP and the results are quite close.