PE Thermal and Fluids System Study Pal (October 2019)

PE_Thermal_Fluids_Examinee said:

Hello Fellow Engineers,

I just got approved to write the PE Thermal and Fluids System exam for October 25, 2019 and looking for a study pal/group to stay motivated and focused during the exam prep. I’ve got a couple of practice problem books that we could go through together, aiming to improve our efficiency from each other’s technique. Should you happen to know any PE Thermal Fluids examinee on Fall 2019 that might be interested, please tag their name(s) then I will try to reach out to them. By the way, I am from the west coast hence on pacific time zone, but open to adjust my time for a study session for an hour or two. All the best to all.

Regards

Click to expand...

Slay the P.E. said:

In a sense, these boards are like a study group.  You can post here any questions that arise during your study and the community typically answers very quickly.

Also, if you're looking for additional practice problems, check out this thread: 

Click to expand...

PE_Thermal_Fluids_Examinee said:

Hi Slay The P.E.,

Thank you for your input.

Potentially asking silly questions, I prefer to save the embarrassment to a person or two.  

Also, I am afraid that to share problems specifics to the forum would be copyright infringement. 

Private study session of a group of 2 to 3 examinees would be a little less troublesome.

Click to expand...

Audi driver said:

There really aren't silly questions. Engineering is difficult and getting the basics down is important. If you can get those questions out of the way quickly, you can move on to the harder topics sooner. Folks here are very willing to help, IME.

Click to expand...

PE_Thermal_Fluids_Examinee said:

Thank you Audi driver, P.E.

Question then from NCEES Practice Exam:

Air, an ideal gas, is compressed and stores in an underground area. Known data are given.. The power required to compress the gas..

Given:

temperature inlet

P2/P1

Compressor Effeciency

mass flow rate

My question, why the formula

”power = mass flow rate (change in enthalpy)”

was used, instead of

”power = mass flow rate (Cp of air) (change in temperature)”?

Answers are 45810 and 46309 respectively.

Click to expand...

Slay the P.E. said:

The energy balance for this configuration is always power = mass flow rate (change in enthalpy). It's just a matter of how you want to calculate the change in enthalpy. The simplest way is to assume constant specific heat so that (change in enthalpy) = Cp (change in temperature). A little more accuracy is obtained if instead of assuming constant Cp you use the air tables which are calculated with variable Cp (this is what they did in the solution). 

At this point people always ask "when do I use constant Cp and when do I use tables?". With constant Cp the results obtained are reasonably accurate if the temperature interval is not very large. But, what is "very large"? In this problem as a first pass we calculate the discharge temperature assuming constant Cp to be about 1330 R. The inlet was 530 R so the temperature range was about 800R, yet the difference in the result (45810 vs 46309) is about 1%.  If the temperature difference is like 1000R I would start to be more careful.  The other criterion is if the problem statement explicitly asks you to use the air tables (e.g. problem 535 in the NCEES sample exam)

This also depends on the gas. As you can see from the figure below, you can almost blindly use constant Cp for of monatomic gases such as argon, neon, and helium, but for something like CO2 you would have to be careful with what value of Cp you're going to pick and use as a constant in your calculations.

View attachment 13747

Click to expand...

PE_Thermal_Fluids_Examinee said:

I appreciate your prompt response Slay The P.E., very well explained. What prompts me to ask the question was because the problem does not mention "air, with variable specific heats" like problem 535 did. That throws me off a little and assumed the constant specific heat is to be used.

Anyhow, I needed more of practice problems like this where it seems very straight forward but actually not. I read a lot about your materials being the most realistic to the actual exam in terms of difficulty. If time permits, I'll plan to purchase your practice problems and solution after I finish covering my current review materials.

Thank you so much Slay The P.E.

Cheers!

Click to expand...

Galalliator said:

@Slay the P.E. i purchased your practice exam and i worked most of the problems, they look frustrating in the begining but i think they have a lot of new approaches.

My question is, i am going to re work your exam again tomorrow then what ?

Do you think your exam is going to be enough or what do i have to do more

Thank you

Click to expand...

Slay the P.E. said:

I’m grateful that you’ve tried our exam and that you’ve found it useful. I assume you’ve also worked the NCEES practice exam.

That being said, I don’t think I’ve ever encountered anyone who only used those two resources for their exam prep. There’s a lot more you can do; although at this point time is quite precious.

At a minimum, I would recommend getting the practice exam from Engineering Pro Guides to get another fresh set of problems:  https://www.engproguides.com/thermalexam.html

I would also shamelessly plug our practice problems books available from our website, but like I said, probably not worth spending the money on the set of books if you only have 3 days to review the 300+ questions in them. ¯\_(ツ)_/¯

Click to expand...

Galalliator said:

@Slay the P.E. yes you right, i can not rely only on two sources.

I did the engineering proguide 72/80 in 5 hours first pass, slay the pe twice,sms twice,ppi practice exam twice but off course not all the problems of ppi exam and sms , they are horrible, i also did ncees 3 times and DTC .

I feel like all the information are messed up now lol.

I have some weak areas that i just know the basics (e.g economics, pipe stresses and joints)

From my experience with fe exam, the ncees has a unique style and no one can ever predict the kind of questions he is getring, they can be pretty easy or horrible.

Thanks for your help,

Click to expand...