# NCEES 2011 Problem 129



## Niiickname (Oct 5, 2016)

I red relevant question and answer.

However I can't understand still and find relevant para. in MERM.

1. How can I know it is saturated steam?

2. I can't understand how second equation was applied.

    I can't find relevant equation from MERM.


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## pmc (Oct 5, 2016)

The question is asking for quality of the steam. This should be your hint that it is saturated steam being withdrawn.  The second equation is the equation for enthalpy of saturated steam that takes into account the quality of the steam.

i.e.:  h=hf+x(hfg)

The key to this problem is that the mixture enthalpy (h, in the above equation) is the average of the water and steam inlet enthalpies.


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## JHW 3d (Oct 5, 2016)

pmc said:


> The question is asking for quality of the steam. This should be your hint that it is saturated steam being withdrawn.  The second equation is the equation for enthalpy of saturated steam that takes into account the quality of the steam.i.e.:  h=hf+x(hfg)
> 
> The key to this problem is that the mixture enthalpy (h, in the above equation) is the average of the water and steam inlet enthalpies.


That's true but there are nuances in the question that suggest why you average:

1. They are both 100lbm/min (equal parts)

2. They have been "thoroughly mixed."

For example if they were not equal parts then you may instead need to take the enthalpy weighted average based on their mass contributions.


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## Audi Driver P.E. (Oct 5, 2016)

How do you know its saturated steam?  You don't.  In fact, it's not.  If it were saturated steam, the quality of the steam would be 1.  Furthermore, the problem says explicitly that you are withdrawing a mixture of water and steam (i.e. thorough mixing).  Steam quality is explained in the MERM (version 13) in section 23-9 paragraph 17 and 23-13 paragraph 32.


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## Niiickname (Oct 9, 2016)

Thanks everyone for the help!


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