# Problem 533 ncees



## mot14 (Mar 30, 2011)

Does anyone know where h (sub d) comes from in MERMs? - the solution they have is 85.8 btu/lbm.....

thanks in advance


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## Clydeman (Mar 31, 2011)

mot14 said:


> Does anyone know where h (sub d) comes from in MERMs? - the solution they have is 85.8 btu/lbm.....
> thanks in advance


NCEES does some strange things on occasion with enthalpy values. What they have done here is interpolated off of the compressed water tables (Appendix 24.D in MERMs). The numbers are as follows:

T (F) P (psia) h

100 3000 75.91

200 3000 174.9

Interpolate to 110 and you get 85.8.

In problems such as these (feedwater heaters) you really should be able to use saturated values. I have worked with problem recently and got it right (and I never use the compressed tables). The answers should never be so close that you end up getting it wrong because of a few enthalpy. I say should, but there are plenty of other NCEES problems were the mass flow rates are very high. In these cases if you do not get the enthalpy very close to what they have you will get the problem wrong.

Also in the NCEES problems there is a lot of interpolation. I hope there is not that much on the test, but I would expect that there will be.

I guess to be safe, if you see a liquid at low temps and very high pressure it is probably best to use the compressed tables (if you remember).


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