# NCEES practice sample exam problem #140



## Saad85 (Feb 28, 2017)

Greetings all,

would you please exapain how the author did the mass balance for question 140 ( heater problem) in NCEES sample test version 2017?

it is not clear for me. Your feedback is highly appreciated.

regards,


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## Ramnares P.E. (Feb 28, 2017)

Can you please post details of the problem?  We all don't have the book.


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## Audi Driver P.E. (Mar 1, 2017)

I suggest a snapshot of the page.


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## Saad85 (Mar 2, 2017)

[No message]


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## Saad85 (Mar 2, 2017)

Audi driver said:


> I suggest a snapshot of the page.










Ramnares P.E. said:


> Can you please post details of the problem?  We all don't have the book.


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## Saad85 (Mar 2, 2017)




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## Saad85 (Mar 2, 2017)

[No message]


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## justin-hawaii (Mar 5, 2017)

Hi Saad85,

I believe you are talking about the cascading feewater heater problem.  The main problem is that you have two unknown mass flow rates, the entering saturated liquid (m_1) and the leaving saturated liquid (m_3).  The problem asks you to solve for m_3.  Since you have two unknowns, you need two balanced equations.  The first equation that you will use are the Mass Balance (m_1 + m_2 = m_3), where m_2 is the entering steam mass flow rate of 100,000 lbm/hr.  The second equation is the energy balance around the heater.  The energy gained by the hot water is equal to the energy lost by the entering saturated liquid and entering steam.  

I put together a quick list of the steps involved in the problem, hopefully this makes sense.  Let me know if you have any questions.  I think during the exam, the main takeaway is that if you have 2 unknowns you most likely need two equations.  

http://www.engproguides.com

View attachment Thermal and Fluids Problem 140.pdf


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## Saad85 (Mar 6, 2017)

justin-hawaii said:


> Hi Saad85,
> 
> I believe you are talking about the cascading feewater heater problem.  The main problem is that you have two unknown mass flow rates, the entering saturated liquid (m_1) and the leaving saturated liquid (m_3).  The problem asks you to solve for m_3.  Since you have two unknowns, you need two balanced equations.  The first equation that you will use are the Mass Balance (m_1 + m_2 = m_3), where m_2 is the entering steam mass flow rate of 100,000 lbm/hr.  The second equation is the energy balance around the heater.  The energy gained by the hot water is equal to the energy lost by the entering saturated liquid and entering steam.
> 
> ...


Thanks a lot for your feedback , your support is highly appreciated


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## Saad85 (Mar 6, 2017)

justin-hawaii said:


> Hi Saad85,
> 
> I believe you are talking about the cascading feewater heater problem.  The main problem is that you have two unknown mass flow rates, the entering saturated liquid (m_1) and the leaving saturated liquid (m_3).  The problem asks you to solve for m_3.  Since you have two unknowns, you need two balanced equations.  The first equation that you will use are the Mass Balance (m_1 + m_2 = m_3), where m_2 is the entering steam mass flow rate of 100,000 lbm/hr.  The second equation is the energy balance around the heater.  The energy gained by the hot water is equal to the energy lost by the entering saturated liquid and entering steam.
> 
> ...


Thanks again for your feedback, would you please explain the mass balance for problem #510 in NCEES sample test?

moreover I am really interested to purchase your mecanical PE thermal and fluid full exam. But does this exam reflect the actual specification of actual exam.

thanks,


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## justin-hawaii (Mar 6, 2017)

Hi Saad85,

I would suggest we start another thread for #510, just to stay on topic and to make the forum more organized.  I will message you directly about the sample exam that I wrote for the same reason.  Thank you!


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## cvanwy02 (Feb 4, 2019)

justin-hawaii said:


> Hi Saad85,
> 
> I believe you are talking about the cascading feewater heater problem.  The main problem is that you have two unknown mass flow rates, the entering saturated liquid (m_1) and the leaving saturated liquid (m_3).  The problem asks you to solve for m_3.  Since you have two unknowns, you need two balanced equations.  The first equation that you will use are the Mass Balance (m_1 + m_2 = m_3), where m_2 is the entering steam mass flow rate of 100,000 lbm/hr.  The second equation is the energy balance around the heater.  The energy gained by the hot water is equal to the energy lost by the entering saturated liquid and entering steam.
> 
> ...


This solution is way easier to understand than the NCEES solution. Thanks @justin-hawaii!


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