2016 NCEES TFS 530

Can someone explain this to me?


 

GR8 PLUMENG said:

Can someone explain this to me?


 

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The turbine has an intermediate extraction stage (state 2), so part of the steam is discharged at state 2 and not all of it makes it all the way to state 3. They want you to calculate the isentropic efficiency of the process from 1 to 2 only. You just apply the definition of turbine isentropic efficiency between 1 and 2.

Thanks for your response. So it appears that they are treating the steam at stage 2 as a super heated steam. Based on the solutions, h(exit) is read from the super heated tables, however, h(es) is read from the Mollier's Diagram. Why is that?

State 2 is indeed superheated steam: The saturation temperature at 96 psia is about 325F; since the temperature is 450F it is unquestionably superheated steam.

From there you are free to use either a Mollier diagram or the written tables to determine 2s. Its really the same information on both. Mollier diagram may be quicker when you are trying to determine the end state of an isentropic process. Mollier is a real time saver here. You can just draw a vertical line from 1 to the constant pressure line of 96 psia and right there you have 2s. Using the written tables would take considerably much more time.

So I don't confuse myself with this type of problem, I just always assume 100% efficiency and calculate h2 theoretical then get actual h2 from the given data you get the efficiency.

I find it more time saver doing this than using a ruler and triangle. As well as its more accurate to solve than to use mollier. Hence, I won't need to worry about accuracy if in case the choices presented so.

Vel2018 said:

So I don't confuse myself with this type of problem, I just always assume 100% efficiency and calculate h2 theoretical then get actual h2 from the given data you get the efficiency.

I find it more time saver doing this than using a ruler and triangle. As well as its more accurate to solve than to use mollier. Hence, I won't need to worry about accuracy if in case the choices presented so.

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The answer choices are very unlikely to be so close together that you would pick the wrong one based on having used a graph. Most wrong answer choices are obtained when you fail to use some unit conversion or due to some other trick.

 I posted a practice problem to illustrate my point of how good a Mollier diagram can be. Do this problem both ways: with the tables and with a Mollier diagram. See which one is quicker.

Slay the P.E. said:

The answer choices are very unlikely to be so close together that you would pick the wrong one based on having used a graph. Most wrong answer choices are obtained when you fail to use some unit conversion or due to some other trick.

 I posted a practice problem to illustrate my point of how good a Mollier diagram can be. Do this problem both ways: with the tables and with a Mollier diagram. See which one is quicker.

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Ok I will try when I got home and I will honestly time both approach.