# Thermo Frustration



## RLies (Jul 16, 2014)

Any tips for thermo problems? I am working through the MERM and I am having a heck of a time finishing the thermo problems. My issue seems to be sorting through the million different equations.

Is there a way to make it easier to know what equations to use in certain situations? I know the obvious answer is to just keep practicing, but I just feel like I'm never going to make it stick.


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## Mike M PE (Jul 16, 2014)

I would suggest getting the FERM and using the formulas in there. More concise (two or three pages).


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## Jonhnny123 (Jul 16, 2014)

Break each cycle down to the individual processes. Get familiar with what physical activity is associated with what process. For example water through a pump is an isentropic process.

The MERM lists each type of process and the main characteristic of it Chapter 25 "Types of Processes" in the 12th edition.

For example in a constant pressure (isobaric) process Q = H2 - H1. In an isentropic process, Q = 0.

When you have a slightly better understanding of that, you should be able to look at that list of formulas and get a better idea of which ones are applicable.

Are you taking a review course? That would be a big help. If not, just keep practicing!


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## Mike M PE (Jul 17, 2014)

Johnny123 hit the nail on the head. The formulas won't do you any good without a good foundation. He's also right about a review course as it helps tremendously with the recognition of which formula to use.


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## SNAPE/SMOTT PE (Jul 17, 2014)

Dr Tom's classroom has an online only version that you work at your own pace. That's what I'm doing. Included is an exam day companion.

Failed twice, and figured I need the review course. I've found out there are a lot of details that I have forgotten about.


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## RLies (Jul 21, 2014)

Thanks, it's getting better the more i do problems.

Regarding power cycles, the practice problems aren't really hard so much as time consuming. Will I need to be able to get really fast at looking up all those values or are the problems on the exam shorter?


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## Jonhnny123 (Jul 22, 2014)

The MERM problems take FAR longer than the exam questions. There are some that can take close to an hour. The best representation of exam problems would be the NCEES practice exams.

With that said, being able to work out the entire power cycle is useful. On the actual exam you'll be given a huge system but only asked to solve one small aspect of it.


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## RLies (Jul 25, 2014)

What tips would you give for when to use the Mollier diagram? I know that I'll be able to save a lot of time if I use it properly, but I never seem to know when to use it.

Also, anyone know where i can find an easier to read Mollier diagram? I'd like to be able to print one off so I don't have to flip in the book so much.


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## SNAPE/SMOTT PE (Jul 25, 2014)

RLies said:


> What tips would you give for when to use the Mollier diagram? I know that I'll be able to save a lot of time if I use it properly, but I never seem to know when to use it.
> 
> Also, anyone know where i can find an easier to read Mollier diagram? I'd like to be able to print one off so I don't have to flip in the book so much.


Ditto! I'm having trouble with this myself...knowing when to use it.

Also, does it help to buy the expanded steam and gas tables?


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## center*ice (Jul 25, 2014)

StillNotA PE said:


> RLies said:
> 
> 
> > What tips would you give for when to use the Mollier diagram? I know that I'll be able to save a lot of time if I use it properly, but I never seem to know when to use it.
> ...


Everyone's different, but I didn't worry about using Mollier h-s diagrams. If you work enough thermo problems using the tables, you'll be able to do them in your sleep. In some instances you might sacrifice accuracy using a Mollier diagram. If you're talking about the psychrometric chart for HVAC problems, however, that's a different story. Learn to use that one well.


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## SNAPE/SMOTT PE (Jul 25, 2014)

There is an NCEES problem that used the Mollier diagram. I tried using the tables, and just couldn't get the same answer. Thermo problem, not HVAC. I was working it last night, and got so frustrated I gave up figuring out how to get the answer using the tables.


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## center*ice (Jul 25, 2014)

StillNotA PE said:


> There is an NCEES problem that used the Mollier diagram. I tried using the tables, and just couldn't get the same answer. Thermo problem, not HVAC. I was working it last night, and got so frustrated I gave up figuring out how to get the answer using the tables.




Is it from the 2011 NCEES Thermal and Fluids Systems sample exam?


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## SNAPE/SMOTT PE (Jul 25, 2014)

center*ice said:


> StillNotA PE said:
> 
> 
> > There is an NCEES problem that used the Mollier diagram. I tried using the tables, and just couldn't get the same answer. Thermo problem, not HVAC. I was working it last night, and got so frustrated I gave up figuring out how to get the answer using the tables.
> ...


I have the 2008, but am told it's the same as 2011. TFS problem 516, steam turbine problem. Finding h2' was what I couldn't determine from steam tables.


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## center*ice (Jul 25, 2014)

StillNotA PE said:


> center*ice said:
> 
> 
> > StillNotA PE said:
> ...


Note that state 1 and state 2 are both at superheated steam. Note also that the turbine expansion is isentropic, hence, s1=s2. I'm assuming you found s1 (1.6263 Btu/lbm-R) from the data given at state 1. Now, go to the superheated steam tables and find the pressure you were given at state 2 (96 psia). Look at the 's' values at that pressure until you locate s2 (remember s1=s2). The corresponding 'h' value shown at p2, s2 is your h2'.


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## SNAPE/SMOTT PE (Jul 25, 2014)

Thanks! I got the s1=s2, I just didn't seem to be able to get the h2' from tables. Thanks so much!


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## center*ice (Jul 25, 2014)

StillNotA PE said:


> Thanks! I got the s1=s2, I just didn't seem to be able to get the h2' from tables. Thanks so much!




You're welcome!


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## SNAPE/SMOTT PE (Jul 26, 2014)

Looks like using MERM requires a bunch of interpolation. Which expanded tables are the best to buy for the exam? I still haven't bought any, but figure I will need to soon.


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## center*ice (Jul 26, 2014)

StillNotA PE said:


> Looks like using MERM requires a bunch of interpolation. Which expanded tables are the best to buy for the exam? I still haven't bought any, but figure I will need to soon.




I used the tables in my thermo textbook from school. They were easy to read and had the units for each property shown on every page (not all tables do that). I don't recall having to interpolate during either of the two times I took the exam. Maybe I just got lucky.


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