# Air table or Cp equation for compressor work



## seeseansky (Apr 14, 2016)

When we calculate air compressor power for isentropic compression, I am little confused whether to use air table or cp equation. 

My understanding is if "ideal gas" is given in problem we should use mdot*cp*deltaT coz cp is constant for ideal gas.

But there is no mention of "ideal gas" in problem we should use mdot*(h2-h1). Where h should be found in air table.

My understanding is correct??

I found in a problem in NCEES 2011 uses cp equation even if "ideal gas" is not given in problem. That is why I am little confused.


----------



## P-E (Apr 14, 2016)

At work I use the ideal gas law.   Might be different at very high temp and/or pressure


----------



## seeseansky (Apr 14, 2016)

They result in different values. That is why I raised up this matter. If the gap between four values in problem is big, it would be no problem. But if the gap is small, for example SMS TF prob no 20, the correct answer would be different. In SMS prob no 20, the solution uses air table h value even if "ideal gas" is given in problem.  

I just wish the gap in real exam should be small.


----------



## Audi Driver P.E. (Apr 14, 2016)

Which problem in NCEES 2011? I'm not exactly sure what you mean. For example NCEES #511 (in TFS) it's easy to just use the formula Mdot*cp*(deltaT) based on the givens.  Cp is a table lookup.  But so would be h.  The amount of error between methods should be below the difference in available answers.  The only caveat I would throw in, is that each table lookup introduces a small amount of error.  Using Cp delta T vs. Delta h, the delta h introduces error twice.


----------



## seeseansky (Apr 14, 2016)

NCEES TF 2011 prob. 522.


----------



## Audi Driver P.E. (Apr 14, 2016)

seeseansky said:


> NCEES TF 2011 prob. 522.


Well, for one, you're not given enough temperature information in that problem to do a lookup of h values, so that should be your clue.  If they want you to use delta h, they will give inlet and outlet compressor air temps.


----------



## Audi Driver P.E. (Apr 14, 2016)

And I should clarify that in that problem they want you to use the ideal gas laws to find T2.  I haven't bothered to calculate it because my tables don't have enough resolution and I don't feel like interpolating!   ), but what is the difference between what you can look up for h values?  Compare cp(delta T) to (delta h).  What do you get?  It should be close.


----------



## seeseansky (Apr 14, 2016)

Thanks!

They do not provide T2 hence the only way to find T2 is to use ideal gas law. So we should treat it the ideal gas even if "ideal gas " is not mentioned in problem. 

The difference can be disregarded considering the differences in four answers.


----------



## Phenomenon083 (Apr 14, 2016)

You can still use the table to find the enthalphies. For 80F find h1 and Pr1. Since pressure ratio is given you can find Pr2. From Pr2 you can calculate the h2 value. For some reason NCEES always solved compressor/turbine/fan problems by using ideal gas laws.


----------



## Cehouse (Apr 17, 2016)

A little late for the exam and I have packed away my study material so I don't have to look at it hopefully ever, but at least for a few months but what I remember:

mdot * Cp * delta T is for polytropic compression

mdot *Cv * delta T is for isentropic compression


----------



## P-E (Apr 18, 2016)

Dude time for decompression


----------



## JHW 3d (Apr 18, 2016)

Say yes to "happy ending?"


----------



## seeseansky (May 27, 2016)

Happy Ending for me!! Thanks everybody....


----------

