NCEES TFS 508 Problem

[nirvick]

Didn't understand how they solved this problem.  Can anyone please explain this problem step by step?

 

[Slay the P.E.]

Didn't understand how they solved this problem.  Can anyone please explain this problem step by step?

Step-by-step solution:

 

[nirvick]

thank you got it.

another question why don't we consider reducing pressure 1.0  MPa that enters desuperheater ?

 

[Slay the P.E.]

Because of the way we have defined the control volume. Section 1 is upstream of the valve. You choose to place the control volume boundary there because you know the enthalpy there.

However, let’s say you draw the control volume so that it doesn’t include the valve. That is, if section 1 is located between the valve and the de-superheater. What would h1 be? well; h1 would be the same, because the throttling process in the valve occurs at constant enthalpy. 

 

[nirvick]

understood thank you

 

[Kohos]

How do you know whether to read h_f at 1MPa vs. h_f at 40C? Thank you

 

[McEng PE]

How do you know whether to read h_f at 1MPa vs. h_f at 40C? Thank you

Because 40C is below the Tsat for the given pressure (1 MPa = 10 bars, Tsat = 179.9C).

 

[Kohos]

Yes, but at 40C, 1MPa>P_sat so wouldn't it still fall under compressed liquid? Hence my confusion..

Thank you!

 

[McEng PE]

Water at 40C at 1MPa is in the subcooled region, so there isn't any significant change in enthalpy with pressure increase at a certain temperature in the subcooled region. Looking at a P-h diagram will help give you a visual. 

 

[Slay the P.E.]

Yes, but at 40C, 1MPa>P_sat so wouldn't it still fall under compressed liquid? Hence my confusion..

Thank you!

If the given conditions (T,p) are such that the thermodynamic state is “compressed liquid” then you could use the compressed liquid table. That table, however, typically only lists moderately high pressures. 1 MPa (10 bar) is too low and is not listed (at least in the one in MERM Appendix 23.Q in MERM13 where the lowest pressure is 25 bar) Therefore, for compressed liquids we use the approximations;

h(T,p) ~ h_f(T)

v(T,p) ~ v_f(T)

 

[Kohos]

Thank you!  I did do those exact steps, went to the compressed tables and couldn't find the answer since it was too low. Appreciate the help from both, will keep this in my notes

 

[John_Arthur]

Hmm... I had a similar error when was trying to fix my old valve and was a bit hard to understand what to do.