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Guest RVincent

Mollier Diagram vs. Steam Tables

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I have encountered two problems (#504 and #524) in the Thermal and Fluids depth portion of the NCEES Sample Questions book that involve isentropic expansion of steam in a turbine. The solutions obviously came from using the steam tables based on the precision of the enthalpy and entropy values. But, it seems to me that it is way faster to use the Mollier diagram than to interpolate values in the steam tables. I solved them both ways, and found that my numerical answers were much closer to the values in the book when using the steam tables than when I used the Mollier diagram. BUT, when using the Mollier diagram I got an answer much more quickly and the numerical answer was close enough that I would have chosen the correct answer.

So, is it the best strategy to use the Mollier diagram for these types of problems?

Thanks,

Randy

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Probably, but don't put it past NCEES to give you answers that are close enough together that the difference between the two solution methods could cause you to select the incorrect answer. I doubt this would happen, but it definitely could. It depends on which Mollier diagram you're using too, the bigger/clearer it is the more precise you can be. I took a big one into the exam in a 3-ring binder that I could unfold and still have it attached to the binder. I didn't have to use it but I wasn't taking your depth module either, which probably requires this kind of solution more often than the HVAC depth module. I'd solve the problem with the Mollier diagram, and use your judgement when you see how close your answer is to the choices given.

Do you hear that echo RVincent? Are we the only two down here in the Mech. section?

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Probably, but don't put it past NCEES to give you answers that are close enough together that the difference between the two solution methods could cause you to select the incorrect answer. I doubt this would happen, but it definitely could. It depends on which Mollier diagram you're using too, the bigger/clearer it is the more precise you can be. I took a big one into the exam in a 3-ring binder that I could unfold and still have it attached to the binder. I didn't have to use it but I wasn't taking your depth module either, which probably requires this kind of solution more often than the HVAC depth module. I'd solve the problem with the Mollier diagram, and use your judgement when you see how close your answer is to the choices given.

Do you hear that echo RVincent? Are we the only two down here in the Mech. section?

Good suggestion, will do.

I think we ARE the only ones here in this section, so thanks for being here to respond!

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I'm glad to help out, I got a lot of help here when I was studying. I can still vividly remember being where you're at, any info you can get about the test from someone who's taken it can really help.

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Hey I still keep in eye on this sub forum!!

But unfortenetly I don't have a copy of the sample test, and thermo was not my strong suit, so I've let Metro lead on these.

Now come up with some decent machine design questions, and I'm all over it. :multiplespotting:

Anyway I agree with Metro, if you have a big enough mollier diagram, and you are faster with it, try it first. I was personally more comfortable interpolating, but that was just me. I do alot of interpolation at work of other charts, so I'm reasonably quick at it.

Keep plugging along RV, and we will all try to help when we can.

John

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Now come up with some decent machine design questions, and I'm all over it. :multiplespotting:

OK, John. I'll keep you in mind for those machine design problems!

Thanks,

Randy

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I was personally more comfortable interpolating, but that was just me. I do alot of interpolation at work of other charts, so I'm reasonably quick at it.

John,

Did you program an interpolation function in your calculator?

Thanks,

Randy

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Randy,

I didn't, because I knew I was taking the MD dpeth, so I wouldn't have too many questions that would require it, and I was also just using the basic TI 30X SII (IIRC) model, and hadn't messed around with programing it (I don't even know if you can). But I do know that some folks who are civil and use HP's would program interpolation functions in. You may want to do some searching on there sub board, and see what you come up with. At work I have a TI85 (oldy but goody) that I have programed in the past, but to be honest I usually just do interpolation on scratch paper, and simple calculations.

I know this doesn't necesarly apply to what we are talking about, but I believe one of the keys to me passing on the first try was making sure I knew how to work problems from the theory on up, and also always keeping track of my units. Plug and chug can be faster, and if you know what your doing it can be just as good, but it helped me to take a few steps into where those plug and chug formulas came from while studying.

Sorry for the rant when all you asked was a simple question, but hopefully it helped,

John

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^^^^^I can't stress enough the importance of carrying your units through the whole problem from start to finish. If you're dilligent about doing this, you'll save yourself from falling into several NCEES "trap" problems. Also, pay very close attention to what units they ask for in the solution. Sometimes they say "what's the pressure in psi?" at the end of the problem statement, and then list the four possible answers as just numerals, with no units written next to them. You can bet next week's paycheck that the answer you would get out of a plug and chug formula that solves for head in feet will be one of the answer choices given. That's not a perfect example, but you get the idea. Many times they're checking to see if you are really reading the problem statements carefully, and the calculations themselves aren't really that difficult.

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^^^^^I can't stress enough the importance of carrying your units through the whole problem from start to finish. If you're dilligent about doing this, you'll save yourself from falling into several NCEES "trap" problems. Also, pay very close attention to what units they ask for in the solution. Sometimes they say "what's the pressure in psi?" at the end of the problem statement, and then list the four possible answers as just numerals, with no units written next to them. You can bet next week's paycheck that the answer you would get out of a plug and chug formula that solves for head in feet will be one of the answer choices given. That's not a perfect example, but you get the idea. Many times they're checking to see if you are really reading the problem statements carefully, and the calculations themselves aren't really that difficult.

Thanks for the advice! From doing the practice problems, I can really see what you mean about carrying units through and really reading the problems.

You mentioned in an earlier post that you brought in a large Mollier diagram for the test. Can you tell me where you got yours? I ordered one from "the other board", and they sent me one poster size...I don't know what they're thinking. That is way too large, especially since you cannot take anything out of your ring binder during the exam.

Thanks,

Randy

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I've had mine for ages, I can't even remember where I got it from. I don't know how big your poster size one is, but consider folding it up and putting it in your 3-ring binder. As long as you can unfold it and use it without unclasping it from the binder you can still use it. It's not considered "loose paper" unless it's detached from the binder. You might also be able to find a printable diagram somewhere on the internet. If so, you could print it to any size you want, if you've got access to a plotter.

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^^^^^I can't stress enough the importance of carrying your units through the whole problem from start to finish. If you're dilligent about doing this, you'll save yourself from falling into several NCEES "trap" problems. Also, pay very close attention to what units they ask for in the solution. Sometimes they say "what's the pressure in psi?" at the end of the problem statement, and then list the four possible answers as just numerals, with no units written next to them. You can bet next week's paycheck that the answer you would get out of a plug and chug formula that solves for head in feet will be one of the answer choices given. That's not a perfect example, but you get the idea. Many times they're checking to see if you are really reading the problem statements carefully, and the calculations themselves aren't really that difficult.

TRUE! Very true and I can not stress that fact enough myself about very careful observation of the working units and WHICH UNITS ARE THEY ASKING FOR!. This is key to doing well in the exam. Also, use the Mollier Chart, it will save you lots of time and I can almost guarantee that you will have no problems...

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I agree that using the Mollier chart will save time and be sufficient. ModernDoug

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Mollier is better because the PE is about finding "most nearly" solutions FAST!

:whipping:

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Hi guys ! Im new in this forum so if you font mind bear with me hahaha.

I have the following problem

What would be the final temperature of an average natural gas at 350F, when the pressure is dropped from 3500 to 1500 psia?

(a) If flowing through an expansion valve.

(B) When expanding through an adiabatic and reversible engine.

Its pretty simple but i have a question reading the values on the mollier chart, i was given an average gravity natural gas chart where ther are no isobaric or isothermal lines for 3500 and 1500 psia or 350 F. I can interpolate (linear) but i doubt it acctually behaves like this. can somebody help me out .( alerady figured out a.Isenthalpic process and b.isoentropic no problem there) . im attaching the diagram.

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