# NCEES Practice Problems



## JoeFromKS (Jan 11, 2007)

I am studying for the 2007 April ME PE exam and was wondering if anyone could help me with this one:

In regards to practice problem #504 in the Thermal and Fluids Afternoon Sample Questions in the NCEES book "Principles and Practice of Engineering - Mechanical Engineering Sample Questions &amp; Solutions", can anyone explain how to find h3?

I understand the problem but I'm not sure if I am suppoosed to use the entropy from point 2, the turbine exhaust pressure at point 3 or a combination of the two to find h3.

Another problem I may be having is that my steam table data for points 1 and 2 do not exactly match the data shown in the solution.

Any help would be greatly appreciated!


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## MetroRAFB (Jan 11, 2007)

I'll scrape the dust off my book at lunchtime and see if I can help you out at all. It's been in the rolling cart I took into the exam room since 5:30 on October 27th.


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## TouchDown (Jan 11, 2007)

If you are looking at answer key, and I'm looking at same version of study book ( I have the NCEES sample problems from 2001 ). Then you are looking at throttling superheated steam, then expanding through turbine.

Check out MERM, page 25-2.

Bulleted items under Types of processes - bottom two are:

isentropic process, where entropy stays same (across turbine since problem stated it's adiabatic, and you're not thottling)

throttling process, where enthalpy is constant.

PS - I never used the Moody diagram or anything, I like the steam tables book, by Keenan (it's just as fast once you get used to the tables, and you get MUCH more accurate numbers.)

SO, initially, the superheated steam has h~1531, so after throttling, you know that h2 = 1531. Then knowing pressure it throttled to, use h=1530 to find S2 (before it enters turbine).

Once you pass through turbine and expand (since adiabatic process) you can assume that S3=S2.. So at final pressure (given), look in that table to find S3.

PS - you'll still find it's barely superheated, but still superheated at S3 and final pressure. If it's not on the superheated table, then you have to find S3 within the pressure in the saturated tables, and you could use it to back-calculate x (mix).

Am I right?

It's been a little while, so I'm rusty. :blink:


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## TouchDown (Jan 11, 2007)

PS - I think it in another problem the steam is not superheated when exiting, you'll also have to have a Temp to find mix (even with knowing S).

Like I said, I'm rusty.


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## JoeFromKS (Jan 11, 2007)

Ok, I think I got it:

I used the steam table to find the enthalpy for state 1 (1530), then for state 2, I used the Mollier Diagram starting at 1530 enthalpy and found where it intersected the 1000 psia line. This gives an entropy of ~1.66 I then follow this line straight down to find where it intersects the 50 psia line which gives me the enthalpy for point 3 (~1183).

Is that right?

Thanks again for all of your help, I really appreciate it!


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## TouchDown (Jan 11, 2007)

Yeah, Mollier, not Moody (sorry)...

And you got it.


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## TouchDown (Jan 11, 2007)

I was frustrated with the NCEES practice problems from the standpoint that they didn't give you all the details of the solutions (if you were attempting to learn or see where you might have gone wrong)...

Vs. the "the other board" study materials - practice exams, which I thought were overly difficult compared with the actual exam, but they did cover all the bases and give you all the information you needed to really understand the solution.

Double Edged sword.

1 you get more representative problems, but little help with solutions

1 you get crazy difficult problems, with great solutions

If you have the "the other board" practice exam (ONLY IF YOU STUDIED MERM though), use it to boost learning, when you get closer to test time, I'd use the NCEES to work on speed. :drive:


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## MetroRAFB (Jan 11, 2007)

> I was frustrated with the NCEES practice problems from the standpoint that they didn't give you all the details of the solutions (if you were attempting to learn or see where you might have gone wrong)...
> Vs. the "the other board" study materials - practice exams, which I thought were overly difficult compared with the actual exam, but they did cover all the bases and give you all the information you needed to really understand the solution.
> 
> Double Edged sword.
> ...


Agreed, I thought the NCEES sample solutions could have included more detailed explanations. My lunch plans got changed, the folks in the office threw a surprise lunch party for me for passing the PE. They were stacked in the break room like sardines and the owner of the company tricked me into walking in there. It was pretty cool, I still don't know how they managed to keep it a secret for 2 weeks. :true:


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## TouchDown (Jan 11, 2007)

Metro - that's Awesome - it must feel good to be appreciated!

Hope it was a good time.


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