Lindeburg Practice Exam P.151

[deerslayersh]

"The net output power from a steam-based Rankine cycle is 250MW. The cycle has steam entering an isentropic condenser at 56°F and leaving as a saturated liquid at 93°F. The condenser pressure is 5psia. The mass flow rate of the steam is most nearby:"

My strategy: 

1. h1, s1 from superheat tables
   
2. solve for quality, X, using s1=sf+Xsfg
   
3. solve for h2
   
4. h3=hf@5psia, v3=vf@5psia
   
5. h4=h3+vf(P4-P3) (convert psi to psf, convert to Btu/lbm)
   
6. W_turb - W-pump = 250MW = mdot(h1-h2) - mdot(h4-h2), (convert MW to Btu/hr),solve for mdot
   

I did all of this and then went to check the solutions. The solutions use the Mollier diagram and come up with a different h2=1018 Btu/lbm which I can't validate. Because the answer choices differ by an order of magnitude, we both chose the same answer, but they aren't particularly close to one another. How are they getting h2 = 1018Btu/lbm?

Also, the temperature at the outlet of the condenser is provided as 93°F, which I am having trouble understanding as well. Shouldn't the temperature be constant as the heat reduction is attributed to the phase change? 

 

[deerslayersh]

I left off part of the problem in the original post:  "...steam enters isotropic turbine at 1100°F and 1250psia, cooling water enters the condenser at 56°F and leaves at 93°F..." Also h4-h3 is the equation for the work of the pump. Sorry for the confusion.

I think I answered my own question about the condenser. The temperatures given apply to the cooling water used to condense the steam, not the steam itself. I am still struggling with the value of h2.

 

[Audi Driver P.E.]

I think posting the diagram would be helpful.