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MizzouMatt

6 min solutions HVAC

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Does anyone know if these problems are a good indicator of the difficulty of the actual test? I made my 1st pass at the afternoon problems and felt pretty good with them 85-90 percent right. Just wondering if this is what to expect for the test or need to prepare a bit more on the HVAC side.

Thanks

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I can't comment on the difficulty rating compared to the test, but I had the opposite experience with Lindeburgs practice test; found the problems very difficult compared to six minute solutions and other sample tests.

I plan to take the NCEES test this Friday, I'm hoping for 80 to 90%. I'll use the remaining few days to practice problem areas, but plan to do no problems or studying after Tuesday.

Ready or not, exam time is upon us. Good luck all!

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I worked the 1st 15 problems of Lindberg and it was pretty tough. They are probably good problems to work but not a good indicator to test readiness. I think I saw that Lindberg said that most people score around a 50 on this test. The problems just had too many steps. They would be 3 problems on the NCEES test.

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I did the NCEES afternoon HVAC problems and found them to be much tougher than the 6ms afternoon. Anyone else agree?

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I thought the NCEES was tougher but I think it was only because I'm weak at power cycles. There weren't many of those in the 6ms book.

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The refrigeration cycles killed me too. I got 30 out of 40 on the NCEES but missed 5 of the 6 cycle problems. Hopefully I will have enough time to figure them out a bit better.

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I thought the NCEES was tougher but I think it was only because I'm weak at power cycles. There weren't many of those in the 6ms book.

Check out the ASHRAE fundamentals chapter on thermodynamics and refrigeration cycles. It is much better than the MERM refrigeration cycle section. It has a good chart showing the cycle in a pressure - enthalpy, a good set of equations for everything that was asked in the practice test. Along with a diagram showing the typical equipment. I read that page (7-8) in 2011 and was able to work all of the problems that I had trouble with in about 15 min.

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Sweet thanks! I'll read up on it tomorrow.

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Is the info you're referring to in the ASHRAE pocket book as well? And would it be helpful for the morning problems? I'm taking MD depth but could use all the help on power cycles I can get. Just don't want to get another $200 book at this point.

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My guess is that the morning section will not have too much on refrigeration cycles. I do not have the pocket guide so I can not tell you if it is in there. The Fundamentals book is a good reference to have but probably not worth buying if you are not taking the HVAC afternoon section.

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I agree. I have the pocket guide and there is some information in the pocket guide on power cycles, but not any more than is already in the MERM.

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Ok thanks. Good to know. I won't bother getting it then.

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http://www.lorencook.com/PDFs/Catalogs/Cookbook_Catalog.pdf

it may be a good idea to bring this for the morning test. There is alot of good fan and duct design rules of thumb and guideline in there.

Here is a link to the refrigeration cycle cheat sheet that I made from the ASHRAE Thermodynamics section. It would have helped me in the NCEES practice test HVAC afternoon section on about 5 problems. let me know if it helps.
/>http://dl.dropbox.com/u/346335/Refrigeration%20Cycle%20Cheat%20Sheet.pdf

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Looks good! It's definitely going into my binder :)

Thanks!

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Yeah your refrigeration cycle cheat sheet looks like it'll be very helpful. Did you do anything similar for the other power cycles? I had thought about it but never got around to it.

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Nope just this one. I may do something tonight if I have time but I doubt it. If I do I will post here.

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Thanks for the chart and all the other info!

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Anytime. The disclaimer on the chart. Please check it yourself to verify its accuracy. I think all of the info is correct but I cant guarantee its accuracy. If you see a mistake please let me know.

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It looks pretty accurate based on the ASHRAE chapter. I do have a couple of general refrigerant cycle questions though.

When trying to find the refrigerant mass flow, do you use Q2-3 or Q4-1? The MERM uses the evaporator mass flow and I haven't seen an equation for the condenser flow.

When adding superheat, is it just an isobaric process? Would supercooling just bring point 3 to the left?

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Metsfan

You can really use 4q1, 2q3, or 1w2 if you know the enthalpy difference for that piece of equipment. The mass flow rates will be consistent through the system so once you figure it out for one portion of the cycle it is the same for all. Super heat on the compressor suction - Pt 1- will keep the same pressure as the evaporator just jump up in the temp. You can read the enthalpy directly from the chart see the two points on the chart for pt 1 , 1 is at sat. 1 is with 20F superheat. I am not as sure for the subcooling though. I would think it would do as you say but i am not sure.

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MizzouMatt, I see now. For some reason I was thinking Qin and Qout were the same, which would in turn give me different mass flow values. Thanks!

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