Adiabatic Mode of Steam Flowing through the valve

[joisy]

Hello All,

can you please assist me with the particular problem. I do have the water steam (non-saturated) which enters the valve at 659 kPa(a) and 171 Deg C. At the outlet it has the pressure only 389 kPa(a). Assuming that proceeding process is adiabatic how to define the outlet temperature?

thanks,
Joisy

 

[joisy]

Assuming that the question is too difficult I will answer on it myself!

It needs to define Enthalpy "H" of superheated steam at the inlet conditions (P1 & T1), and then find what temperature complies to "H" if the pressure is 389 kPa.

hope this is correct & can be useful for someone
cheers,
Joisy

 

[quark]

Joisy!

The procedure seems to be ok for a perfect adiabatic condition. The latent heat of steam at low pressures is high, so a portion of the sensible heat from high pressure steam provides this and the final temperature of low pressure steam will be less than that of high pressure steam.

In your case, vapor enthalpy at 659 kPa is 2760.19 kJ/Kg and at 389 kPa is 2736.78 kJ/Kg. There is a difference of 23.41 kJ/Kg. Specific heat of steam at 659 kPa and 171 deg.C is 2.395 kJ/Kg deg.C. So there should be a temperature drop of 23.41/2.395 = 9.77 deg.C approximately.

So final temperature will be 161.23 deg.C (again I say approximately)

Trial and error by constant enthalpy method shows 162 deg.C

I don't think this question is too difficult for the folks like Montemayor, TD2K, 25362 and MJCronin etc. But I think something is fishy:-(

I will come to know only if this post gets deleted. (and this is happening regularly now a days)

Good Luck,

 

[hacksaw]

i get 162.5 oC (numerical soln)...

 

[TD2K]

Joisy's answer is correct, the expansion of steam across a valve is considered isenthalphic. Simply determine the enthlaphy of the steam at the inlet pressure and temperature and then determine for the outlet pressure, what temperature has the same enthalphy.

Unfortunately, my SS that does this isn't here, will add in a reply tonight but my guess is that Quark and Hacksaw are right.

 

[TD2K]

162.11 deg C using my steam table program for your information.

 

[joisy]

Guys,

and what tables have you used to define enthalpy? The ones I have have very inconsistent & give the significant mistakes during interpolation?!

thanks,
Joisy

 

[quark]

Why can't you download the steam tab from

http://www.chemicalogic.com/steamtab/companion/default.htm

?

Regards,

 

[TD2K]

One free Excel add-in is at

http://www.cheresources.com/iapwsif97.shtml.

I also have a steam table program from Archon Engineering, very nice package IMO. It's about $40.