NCEES TFS practice exam problem 533

[Mech_Engineer]

Hey folks, 

Did a quick search and didn't see anything on this. For those that have the NCEES TFS practice exam:

When solving for T3 (out of regenerator, before burner), shouldn't you use the actual T5 temperature out of the turbine rather than the ideal outlet temp (turbine efficiency is given)? Looks like the solution uses ideal temps to solve for T3. I used the actual T5 temperature and I get a lower cycle efficiency from what the solution is showing as the answer. I get it for the compressor as it's ideal, but not understanding the turbine since it gives an efficiency. Any help?

 

[Slay the P.E.]

The table of temperature values are all actual values. When they say "Station 5" they mean that is what a probe is registering there. The provided turbine efficiency is unnecessary for this problem and is a "distractor".

 

[Mech_Engineer]

The table of temperature values are all actual values. When they say "Station 5" they mean that is what a probe is registering there. The provided turbine efficiency is unnecessary for this problem and is a "distractor".

Thanks, Slay. You've been a great help.

That's interesting. Now, I'm wondering that if they give the values, if we should always take those as actual, but if not given, work the problem isentropically then find the actuals?

 

[squaretaper LIT AF PE]

Thanks, Slay. You've been a great help.

That's interesting. Now, I'm wondering that if they give the values, if we should always take those as actual, but if not given, work the problem isentropically then find the actuals?

Yep. that's when you use the efficiency equations. See eqn. 26.17 in MERM where h₂' is the actual enthalpy. Rearrange and solve.

 

[Slay the P.E.]

Thanks, Slay. You've been a great help.

That's interesting. Now, I'm wondering that if they give the values, if we should always take those as actual, but if not given, work the problem isentropically then find the actuals?

Correct. If T5 had not been given, then you would need to calculate it.

 

[Mech_Engineer]

Awesome. Thanks again. So, temps given. Use that as actual. If not, calculate ideal, then find actual via given efficiency. This helps a lot!