# Question: Enthalpy Saturated Water



## Maximum_Entropy (Apr 27, 2020)

I am having trouble figuring out how the authors arrived at a solution on the NCEES Mechanical Thermal and Fluid practice exam (978-1-947801-08-00). 

For reference, my review and use of the practice test is in preparation for the CBT exam format where the only reference allowed is the electronic PDF of the NCEES Mechanical Reference Handbook. For this reason, my studying has been heavily relying on learning flow and gems hidden in the reference manual, albeit it is not my preferred source.

*Question 48 *of the practice exam, shown on the attached picture, is a fairly simple looking problem with just performing a energy and mass balance. 

My confusion lies in how the author was able to obtain the enthalpy of the "1MPa, 40 C" attemporator water.  The author just throws down the enthalpy, h2, as 167.57 kJ/kg.

When I solve this like I normally would and just use MATLAB Xsteam US to look up the properties or the keen and kean tables I have no problem getting the enthalpy of the water at ~167 kJ/kg. I don't see how they got it from the limited steam tables provided in the NCEES Reference Manual. 

Does anyone have recommendation of an alternative from the table method to determine the enthalpy of the attemporator water? 

The closest thing I can get it by taking the difference of hv and hfg of table 6.3.4. at the temperature of the water but regardless of the pressure. I excerpted the NCEES reference manual table below. I cannot find justification or explanation of why this would work. Alternatively if some one can help explain why taking the difference of h_v and h_fg at the 40 deg. sat liquid that would help too.

Thanks in advance. 

-Connor


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## MikeGlass1969 (Apr 27, 2020)

Download the latest revision of the NCEES Mechanical Engineering Manual....  

hf = 167.53 KJ/kg  is in my manual page 325 chapter 6.3.4


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## Maximum_Entropy (Apr 27, 2020)

Helpful. I was working out of rev 1.0. This makes me feel better then. I don't frequently work in SI so maybe that is why it didn't jump out to me as a very wrong number. Looking  a little closer at mine it appears to be an issue where the specific volume values were copied over one column until T=55 C. 

thank you @MikeGlass1969

It's small errors like this I keep running into in the reference book that have thrown me for a loop with some topics...


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## Audi Driver P.E. (May 5, 2020)

Maximum_Entropy said:


> Helpful. I was working out of rev 1.0. This makes me feel better then. I don't frequently work in SI so maybe that is why it didn't jump out to me as a very wrong number. Looking  a little closer at mine it appears to be an issue where the specific volume values were copied over one column until T=55 C.
> 
> thank you @MikeGlass1969
> 
> It's small errors like this I keep running into in the reference book that have thrown me for a loop with some topics...


Keep in mind, when the solution was drafted up the authors used a commonly available reference for the steam tables, and I think it was the Kenan and Keyes and you could bring in any reference you felt accurate. The CBT should be based on what they include in their reference.


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