Jump to content


Photo

Steam Enthalpy Maximum?


4 replies to this topic

#1 2bsss

2bsss

    Intern

  • Members
  • 24 posts
  • Discipline:Chemical

Posted 06 December 2011 - 11:23 PM

There isn't a chemical specific forum so I'll try it here; looking at steam tables for saturated steam, it looks like the enthalpy (H) reaches a maximum at about 450 psia (as does the internal energy, U) and decreases as pressure/temperature increases at saturation after that. Thermodynamically, why is this and what does it mean?

#2 CbusPaul

CbusPaul

    CbusPaul

  • Senior Member
  • 329 posts
  • Gender:Male
  • Location:OH
  • Discipline:Sorely Lacking

Posted 07 December 2011 - 03:44 PM

There is a chemical specific forum. Try posting there and I'll do some research on an answer for you.

#3 pbrme

pbrme

    You look like I...

  • Supporting Member
  • 8,428 posts
  • Gender:Male
  • Location:The PNW
  • Discipline:Mechanical

Posted 20 December 2011 - 01:08 AM

This is an appropriate question for the mechanical thread as it pertains to thermodynamics. A good visual for the steam tables is the Mollier diagram (make sure you aren't looking at the T-S diagram). On the Mollier diagram you'll see a line indicated as the saturation line, this is the dividing line between the saturated steam and the super heated steam regions and respective tables. You can see the maximum Enthalpy for saturated steam (under the saturation dome) on the diagram matches the data in the tables. This visual is useful for "seeing" what happens as steam moves from one stage to another in a thermodynamic cycle. To answer your question: It decreases in the left hand region of the Mollier, after the peak, because this is the nature of steam (if kept at saturation) and the first law. Remember h = u + Pv


The T-S diagram is more useful (IMO) but harder to read if you're not as familiar.

#4 2bsss

2bsss

    Intern

  • Members
  • 24 posts
  • Discipline:Chemical

Posted 22 December 2011 - 02:01 AM

What I am having a hard time grasping is that saturated steam at 1000 psia has less enthalpy than saturated steam at 450 psia. Does this mean that it takes more energy to move a quantity (say one pound) of saturated steam at 15 psia to saturated steam at 450 psia than it does to raise the same quantity of saturated steam (initially at 15 psia) to 1000 psia? Or the reverse, is more energy available (lets say for heating another fluid) from saturated 450 psia steam than from 1000 psia saturated steam?

#5 pbrme

pbrme

    You look like I...

  • Supporting Member
  • 8,428 posts
  • Gender:Male
  • Location:The PNW
  • Discipline:Mechanical

Posted 03 January 2012 - 09:42 PM

Essentially, this is just the very nature of steam as the information given in the steam tables is based on empirical data. Without going into a lengthy thermodynamic discussion, the answer to your question is yes.

Edit: I also forgot to mention "If keeping it in the boundaries of the steam dome".




0 user(s) are reading this topic

0 members, 0 guests, 0 anonymous users

=