Liquid nitrogen to gas

[chunia]

Hi,

How to calculate how many BTUs it takes to convert one gallon of liquid nitrogen (LN2) into gas?
We use heat exchanger with 280F hot water. Output gas temperature 70F

TIA

 

[Unotec]

At what pressure?

<<A good friend will bail you out of jail, but a true friend
will be sitting beside you saying ” Damn that was fun!” - Unknown>>

 

[chunia]

95 to 100 psi

Thanks

 

[Unotec]

Have you looked at the Nitrogen phase diagrams yet?

<<A good friend will bail you out of jail, but a true friend
will be sitting beside you saying ” Damn that was fun!” - Unknown>>

 

[chunia]

No, where I can find it?

Thanks

 

[Unotec]

Google it, probably there or in the library. Just look for a pure substance phase diagram (unless you have a mix). That will answer your question. If not let us know and we'll help more

<<A good friend will bail you out of jail, but a true friend
will be sitting beside you saying ” Damn that was fun!” - Unknown>>

 

[Unotec]

hi Chunia, any luck?

<<A good friend will bail you out of jail, but a true friend
will be sitting beside you saying ” Damn that was fun!” - Unknown>>

 

[dcasto]

go here

http://webbook.nist.gov/chemistry/

 

[Apakrat]

http://en.wikipedia.org/wiki/Nitrogen

At 74th year working on IR-One2 PhD from UHK - - -

 

[chunia]

Guys,

Sorry for not getting back here sooner, just was busy with electrical problems.
Thank you for pointing me to the right direction, Google is really good source of information, but I did not find an answer to my question. I will keep looking.

 

[quark]

You might have overlooked dcasto's post. Go to NIST webbook and check the saturation data of nitrogen. The enthalpy difference between the gaseous state at required condition and the liquid condition gives you btu required per lb.

 

[chunia]

According to the table out of NIST webbook it takes 85 Btu to convert pound of liquid Nitrogen to gas (1 deg. delta)
Am I correct?

 

[Unotec]

At 70ºF and 100 psi, are you sure you have liquid nitrogen?

<<A good friend will bail you out of jail, but a true friend
will be sitting beside you saying ” Damn that was fun!” - Unknown>>

 

[chunia]

Unotec,

It just for converging from liquid to gas. Now I need to warm up the gas from -319F to +70F at 100psi.

 

[Unotec]

And your goal is to keep the pressure constant? I am sure you have considered it already, but with the volume expansion you will have a pressure increase.
If I am not mistaken, the 85BTU you came up with will just be the energy for the phase change, right? You still need the energy to go from -319ºF to boiling (sub cooled liquid) and the energy after boiling to 70ºF (superheated gas)

Have you considered those? (I can't open the NIST page for some funky reason)

<<A good friend will bail you out of jail, but a true friend
will be sitting beside you saying ” Damn that was fun!” - Unknown>>

 

[chunia]

Yes, you are absolutely correct. Thanks. I saw the formula somewhere ( Btu for warming gas) just need to find it.

 

[quark]

Seems to be a problem with your data. Enthalpy of LN2 at -319C is -51.79 btu/lbm and enthalpy of N2 at 100 psia and 70F (isobaric process) is 130.73 btu/lbm. So, you require 182.72 btu/lbm of N2, approximately.

 

[25362]

Take the Btu/lb estimated by quark and multiply it by 6.71 lb/USG (@ 100 psia and -319^oF) to obtain the requested Btu figure for one gallon of liquid nitrogen.

 

[chunia]

Thanks to everybody for the inputs.

Warming the gas:

Q= 1.08 X CFM X (t1-t2)

1 gal of liquid nitrogen = 93.11 cubic feet

Q= 1.08 X 93.11 X (319+70) = 39117 Btu/gal of liquid or Btu/ 93.11 cfm of gas

Latent heat of vaporization
86.6 Btu/lb = 583.5 Btu/gal

Total: 39700 Btu/gal

According to Quark it suppose to be 182.72 Btu/lbm X 6.7347 = 1230.6 Btu/gal

I made a mistake somewhere, I think in the formula….

 

[quark]

Yup, and more than one.

First, the formula deals with flow rate and not volume. Secondly, the formula is valid for atmospheric pressure (even you need to revise is for change in atmospheric pressures due to change in elevation). Third, it assumes sensible heat at 70F and thus valid for temperatures not far above and below it. Fourth and not very critical one is that it is for air.

Still, since air is mostly nitrogen, considering 0.24btu/lbF specific heat and .075 lb/cu.ft air density,
0.24x.075x93.11x(319+70) = 651.95 btu/gal, plus 583.5 btu/gl of latent heat equals 1235.45 btu/gal