#504 ncee 2001 THERM PM

[Yingli]

Hello,

Does some one can help me on 504# therm PM of 2001 NCEE, how to get h=1530.8, s=1.6698? at 5000psia and 1200 F ? on which table or diagram? I only have steam table and Mollier diagram on MERM?[ I am preparing to take exam on therm PM, Do I need more reference?

Thank you

Yingli

 

[JoeysVee]

I don't have the 2001 NCEES sample exam with me here at work (it's at home) but my guess is you'll need a book of steam tables...something simular to...

http://www.amazon.com/Thermodynamic-Proper...9527&sr=8-5

 

[Yingli]

I don't have the 2001 NCEES sample exam with me here at work (it's at home) but my guess is you'll need a book of steam tables...something simular to...
http://www.amazon.com/Thermodynamic-Proper...9527&sr=8-5

Thank you very much for the infromation, could you please email me at [email protected], let me kown if it directly get from table without any calculation, may I get your phone number?

thanks,

Yingli

 

[buick455]

Use the Mollier diagram, page A-53.

5000 PSIA at 1200 F, h = 1531 Btu/LBM, s=1.48 BTU/Lbm - R not 1.6668

Hello,
Does some one can help me on 504# therm PM of 2001 NCEE, how to get h=1530.8, s=1.6698? at 5000psia and 1200 F ? on which table or diagram? I only have steam table and Mollier diagram on MERM?[ I am preparing to take exam on therm PM, Do I need more reference?

Thank you

Yingli

 

[Yingli]

Use the Mollier diagram, page A-53.
5000 PSIA at 1200 F, h = 1531 Btu/LBM, s=1.48 BTU/Lbm - R not 1.6668

Thank you, so if s=1.48 btu/lbm, then what is the correct h3i? , the answer W=89,780 kw is wrong?

Thank you very much,

Yingli

 

[JoeysVee]

No....89,780 is correct...

I worked this problem a little different than the book although their answer is correct and even quicker.

First you must realized there's 3 different things that happen...step 1, 2, and 3.

First, the steam is at 5000PSIA and 1200 F which gives you h=1530.8 and s=1.50. Then the steam is throttled. h is constant through a throttling process so h1=h2.

The second stage is at h2 = h1= 1530.8 and 1000 psia, so go to the tables and get s at these conditions...that gives you an s =1.6698. Then it goes through the turbine which is a constant entropy process so s2 = s3.

The third stage is at s3 = s2= 1.6698 and 50 psia which at these conditions go to the tables and get an h =1182.6

Then use eq 28.18 out of the MERM to get the actual h (since efficiency is given).

Then the output power is 1,000,000*(h1 - h(actual)) then convert to KW gives 89,780.

To get the real accurate "h" and "s"'s in this problem you need a steam table book like the one I linked above.

I would prefer not to give my number but can give my e-mail to you...although I would prefer to answer your questions here on the forum in case someone else has the same question.

 

[Yingli]

No....89,780 is correct...
I worked this problem a little different than the book although their answer is correct and even quicker.

First you must realized there's 3 different things that happen...step 1, 2, and 3.

First, the steam is at 5000PSIA and 1200 F which gives you h=1530.8 and s=1.50. Then the steam is throttled. h is constant through a throttling process so h1=h2.

The second stage is at h2 = h1= 1530.8 and 1000 psia, so go to the tables and get s at these conditions...that gives you an s =1.6698. Then it goes through the turbine which is a constant entropy process so s2 = s3.

The third stage is at s3 = s2= 1.6698 and 50 psia which at these conditions go to the tables and get an h =1182.6

Then use eq 28.18 out of the MERM to get the actual h (since efficiency is given).

Then the output power is 1,000,000*(h1 - h(actual)) then convert to KW gives 89,780.

To get the real accurate "h" and "s"'s in this problem you need a steam table book like the one I linked above.

I would prefer not to give my number but can give my e-mail to you...although I would prefer to answer your questions here on the forum in case someone else has the same question.

Thank you so much, it is very clear steps!!

I am still don't have clear concepts on the thermo, the reason I chossing thermo because I am better on the fluid.

Mollier diagram has two constant temperture lines, one for constant tem, another for superheated tem, so, when I shoud use constant tem line, or superheated tem line? if question did not say that steam is or not superheated, which line does need be used?

Another question for #523, how to get h3=70 btu/lbm, at p3=60 psia and t3=102 F, h can be found by tem and pressure, when should using by tem, or when shoud using by pressure?

I feel so behind for the exam, I have taken two weeks off for preparing this exam, so far I am so stressed.

 

[JoeysVee]

Those are some great questions! I'm not so sure I can answer them.

As far as the mollier chart question...I don't want to bring a large mollier foldable chart to the exam as I don't think it is allowed so the only choice is the tiny 8.5 x11 mollier chart in the MERM, which is great for a quick rough answer but for a very accurate answer that tiny mollier chart won't do it. So I have decided to use a book of steam tables. I have gotten pretty good with the book so it's almost as quick as the mollier table. My books of steam tables are tabbed. Since I have been using the book of steam tables, I have abandoned the molllier diagram. I also noticed the mollier chart has 2 temp lines and like you, I'm also not sure when to use which set of lines, but again, it's a non issue for me since I use the more accurate tables. Maybe someone else can tell you when to use the different temp line sets.

I think the correct answer to your 2nd question is...If you have a compressed liquid (not saturated) then you can assume it isn't compressible...meaning we assume the pressure has no effect on the enthalpy. So in this case H2O at 60psia and T=102F is unsaturated water so we assume the enthalpy is not changed by pressure (as long as it doesn't become saturated) so we go to the tables and find steam at T=102 and use the sat liquid enthalpy. When a liquid is compressed (not saturated) the enthalpy changes with temp not pressure so you'll always use the saturated enthalpy value at the temp given. I hope this is correct...this is why I use T and not P in this question. Maybe others can chime in and make sure my way of thinking on this is right.

I hope that helps a little anyways!

 

[Yingli]

Thanks again, I have ordered the steam table book that you mentioned this morning , wish I can get it before next Thursday. Your explain is very clear, it is a big help to me.

Yingli

 

[JoeysVee]

Great! If you have any other questions, just post them...that's what I do