Steam Table question

[zakbos]

I am working q530 on the 2016 thermal fluids practice exam. 

The question is asking for the turning efficiency from State 1 to state 2. At state 1, p=900 psia, T=900F. At state 2, P=96 psia, T= 450. I get h1 = 1452, s1=1.6262 and h2(actual) = 1254 from superheated tables. However, the solution lists h2(isentropic) as 1201.24. I do not understand how that number is calculated and there is no detail in the solution. Does anyone know the detail process to calculate that number? 

Thank you very much for the help!

 

[MikeGlass1969]

The isentropic is a chart look up.

 

[zakbos]

@MikeGlass1969 Which chart are you referring to? That value seems to be outside the bounds of the tables(both superheated and saturated at 96psi.)

 

[MikeGlass1969]

Your Mollier Diagram..  A71 in the Appendix.

 

[zakbos]

Got it, thanks so much Mike. 

Is there a quantitave method to calculate it or is the chart look up the only method?

 

[MikeGlass1969]

For the exam..   I would only recommend you to use the Mollier diagram.  It's faster and everything is right there on one page.

 

[Vel2018]

I am working q530 on the 2016 thermal fluids practice exam. 

The question is asking for the turning efficiency from State 1 to state 2. At state 1, p=900 psia, T=900F. At state 2, P=96 psia, T= 450. I get h1 = 1452, s1=1.6262 and h2(actual) = 1254 from superheated tables. However, the solution lists h2(isentropic) as 1201.24. I do not understand how that number is calculated and there is no detail in the solution. Does anyone know the detail process to calculate that number? 

Thank you very much for the help!

The h2 you get from super heated steam tables is the h2' (actual) as the problem given you the actual conditions. 

Now calculating the theoretical 100% efficiency. You use S1=S2

Then theoretically, the condition of state 2 should be in saturated pressure tables. You would go to saturation pressure of 96psia and calculate the quality.

Then you get h2(theoretical). Plug those into equation and you get the isentropic efficiency.

If you use Keenan and Keyes tables, you should get h2 theoretical with value 1201.95 and quality should be about 1.0167.

Hope this helps.

 

[Vel2018]

For the exam..   I would only recommend you to use the Mollier diagram.  It's faster and everything is right there on one page.

I agree only for some particular pressures, and only if you are comfortable using the most 11x17, unless you want waste time flipping that huge paper and folding it back after solving. 

In some cases mollier chart are faster, but not all the time. Specially if pressures are over 2500psi, I would just go to the tables.

 

[Slay the P.E.]

I agree only for some particular pressures, and only if you are comfortable using the most 11x17, unless you want waste time flipping that huge paper and folding it back after solving. 

In some cases mollier chart are faster, but not all the time. Specially if pressures are over 2500psi, I would just go to the tables.

@zakbos see the thread linked below for a discussion comparing both approaches