CP,air = 7.05 Btu/lbmole-oF.
Sunday, April 15, 2007
HW #5, P7 - Waste Heat Steam Generator - 6 pts
At steady-state, water enters the waste heat recovery steam generator shown in the diagram at 42 psia and 220oF and exits at 40 psia and 320oF. The steam is then fed into a turbine from which it exits at 1 psia and a quality of 90%. Air from an oven exhaust enters the steam generator at 360oF and 1 atm with a volumetric flow rate of 3000 ft3/min and exits at 280oF and 1 atm. Ignore all heat exchange with the surroundings and any changes in potential and kinetic energies. Determine the power developed by the turbine in horsepower.
CP,air = 7.05 Btu/lbmole-oF.
CP,air = 7.05 Btu/lbmole-oF.
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11 comments:
dr B,
i cant seem to find a value for V hat, which i thought would be necessary for the computation of m4 dot and hence m1 dot.
y,
At which state are you having trouble with finding Vhat? I looked up Vhat in the thermodynamic data tables using the information for each state. If you read the problem statement carefully, there is at least 2 pieces of info for each state. Let me know at which state V hat is troubling you.
I had problem finding the V hat for air in point 4 since we do not have the tables for air. Do we take the sum of 0.79 of N2 and 0.21 of O2?thanks
Well, I used the Excel plug in and it does have thermodynamic tables for air. Remember that if you don't have the tables first check to see if you can assume the gas is ideal otherwise use an advanced equation of state. The molecular weight of air is 28.97 lbm/lbmol. Does that help?
sparticus,
oh got it..thanks a lot=)
Y:
Treat the air as an ideal gas and use the IG EOS.
Sparticus:
Thank you once again. I like it when you guys answer each other's questions.
my excel plug in doesn't seem to work. could anybody possibly tell me delta Hhat for Hhat5 - Hhat4 (the air).....you need this right to find the mass flow rate of steam?? if i am wrong, please correct me
Rock:
Use the Cp of air = constant = 7.05 Btu/lbmole-degF. Then, deltaH = Cp * deltaT.
Yes you need to know how much heat LEFT the air because all of it went INTO the water. Use the Q in the 1st Law to determine the mass flow rate of the steam. Then, apply the 1st Law to the turbine to find the shaft work (actually power).
I am having trouble finding mdot1.
I am using mdot1= -mdot4(Cpair(dT)/Hhat2-Hhat1)
for mdot4 I am using Vdot4/Vhat4
the combined equation reads
mdot1= -(3000/20.66674)*[(7.05*-80)/(1196-188)] = -81.1702 lbm/min
please advise
Are you single...
I am pretty certain that your problem has to do with units. More specifically, you used a MASS flow rate and a MOLAR heat capacity. You just need to work the molecular weight of air into your analysis in the right place and you will get the right answer. MWair = 28.97 lbm/lbmole.
I am happily married :)
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