# Variable specific heat & constant specific heat



## nirvick

Hi,

Can anyone help me to figure out how to chose between this two Variable specific heat &amp; constant specific heat? I know it's a silly question.But still facing a lot problem when I do the solution for nozzle/diffuser or compressor/turbine problem? Can anyone give me a guideline what specific word in the question I should look for to use Variable specific heat &amp; constant specific heat? or what is the easy methodology to understand that.

Thank you

Karim


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## Sheshtawy

I think it all depends on the accuracy of the results. I don't think any specific wording in a problem would guide you to either one (Unless it clearly says: assume constant specific heat).

Technically, specific heat is always variable and you should always use the specific heat at the given temperature. But this is not always simple nor necessary.

If you check the air tables, you'll find that the air specific heat only changes about 0.5% every 100 F at lower temperatures and around 1% every 100 F at higher temperatures. I don't think that NCEES is evil enough (are they? IDK) to include two answers that are within 5% of accuracy when it comes to problems requiring looking up specific heat.

Bottom line, I'd familiarize myself with the air tables, know how to get the specific heats at different temps, solve some problems with both variable and specific heats and compare the results, and not worry about it!


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## nirvick

Got it. thank you


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## Slay the P.E.

Perhaps the key to this (as it applies only to the PE exam) is found by looking at problems 533, 534, and 535 of NCEES's TFS practice exam.


In problem 533 they explicitly tell you to use constant specific heat (and they even tell you which value to use).

In problem 535 they explicitly tell you to use variable specific heats (i.e. use the air tables).

In problem 534 they don't tell you what to use. If you look at their solution, they went with the air table. What would happen if you'd gone with constant specific heat? If you do, and use _Cp_=0.24 Btu/lbm (not the best value to use, but.. lets play along) you get an answer of 46,264 hp. Their answer is 45,800 so you'd be off by 1%. The other answer choices are off by WAY more than that. You'd still pick the right answer, (D).

So, they're not that "evil" to give you answer choices that are within 1% of each other. You're safe going with constant _Cp_.


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