ASHRAE Superheated tables

Slay the P.E. said:

You can find that in the back of some thermodynamics textbooks.  If you don’t have any, then use a P-h diagram which is more easily available than a table.

We have one here for free https://www.slaythepe.com/free-resources.html

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MikeGlass1969 said:

If you must have a table the iiAR.org ammonia refrigeration data book would have this information.   BUT it is expensive.

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P-E said:

I looked in my fundamentals handbook 2005 chapter 20.  There's a p-h graph that shows superheated region for R717 aka ammonia. 

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sayed said:

the P-h diagrams don't have entropy. It makes me wonder how the problem was solved. This was Q6 in chapter 33

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Slay the P.E. said:

P-h diagrams do have lines of constant entropy. You use them to represent the compression process. The one in our website is color coded so you can find the constant entropy lines easily.

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sayed said:

the P-h diagrams don't have entropy. It makes me wonder how the problem was solved. This was Q6 in chapter 33

this is insane. $2500 for a book?

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sayed said:

thanks

not sure how i missed that. I guess that's something that the ppi class didn't go over (using s on p-h diagram)

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Slay the P.E. said:

Please don't walk into that test room without knowing how to represent a simple vapor compression refrigeration cycle in a P-h diagram. This is an absolute must for both the Thermal Fluids & Systems and the HVAC&R exams.

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MikeGlass1969 said:

People make hundreds of thousands of dollars off those PSM and RMP books, every year....   $2500 seems like a good investment

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NHEngineer037 said:

ASHRAE Fundamentals Chapter 30 has all the charts you need for refrigerants... all of them.

When I start a refrigeration problem, the first thing I do is sketch the shape of the process that you would on the P-H curve, on my sheet of scrap paper.  You know...a rectangle with the right side of it sloped.  Then as I read the problem, I quickly jot on the diagram the temperatures given, the pressure (corrected for psia if given in psig) and make note of superheat or sub cooling by putting a little tick on the drawn box with an arrow pointing to it saying something like "20 F superheat".  Depending on the question, I figure out what enthalpy values I need to figure out and I write them out on my diagram.  Then I find the appropriate chart in ASHRAE Chapter 30 and just figure it out.  The quick napkin sketch makes messing up or forgetting some provided information a non-issue.  They are pretty simple once you get the hang of it.

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namod65 said:

Pretty much exactly what I've been doing. I have a digital copy of the the Fundamentals and printed out the refrigeration diagrams chapter and bound it separately for quicker use. I can't imagine you'll need anything more than this. It even has the superheated data tabulated for R-134a which could make it easier to find those enthalpy points at the compressor. It's sometimes hard to get accurate when following the constant entropy lines up to the compressor discharge.

I hope there's a ton of these problems on the test Friday, like on the practice exams. They're super easy once you do a few.

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