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Thermodynamics Enthalpy of water

  1. Feb 2, 2014 #1
    1. The problem statement, all variables and given/known data
    Liquid water at 212ºF and 1atm has a specific internal energy of 180.02Btu/lbm. The water is transformed into steam at 400ºF and the pressure of 100psi where it's specific volume is 4.937ft^3/lb and it's specific enthalpy is 1228.4Btu/lb. Determine the variation of internal energy and the variation of enthalpy of the process.

    2. Relevant equations

    3. The attempt at a solution

    So I know the specific internal energy in the liquid state (180.02Btu/lbm), and I can calculate the specific internal energy in steam (h=u+Pv ⇔ 1228.4=u+6.804*4.937) which gives me 1194.80. But now I'm stuck... I have the variation of the specific volume (vsteam-vliquid) , the variation of pressure (6.804atm-1atm) and the variation of specific internal energy. So, I can get the variation of the specific enthalpy(h). But how do I get the mass to get the variation of enthalpy(ΔH)? (if what I said is correct of course).

    Thank you for your time and patience
  2. jcsd
  3. Feb 2, 2014 #2
    You had the right idea in the first part, but you need to express the pv part in BTU/lb. Right now, the units are handled incorrectly.

    As far as the second part is concerned, you need to make use of the pressure and specific volume of liquid water at 212 F. The density is about 62.4 lbm/ft3 and the pressure is 14.7 psi.
  4. Feb 2, 2014 #3
    I've corrected the units, I think. Converted 4.937ft^3/lb to Btu. 1ft^3=1.027Btu. It gave me 5.070299.
    So, instead of using the 1atm I'll use the 14.7 psi and the specific volume I calculate it from the density you gave me to use in the (ΔPV) part? But I still don't get it how will I found the mass to convert it to ΔH and ΔU
    Last edited: Feb 2, 2014
  5. Feb 2, 2014 #4
    This conversion is incorrect and also irrelevent.

    The initial specific enthalpy is 180 + pv, where the pv is expressed in units of BTU/lbm. You know the final enthalpy already, so you can get the change in enthalpy.

    The final internal energy is calculated the way you did it, but make sure that the units of the pv are, again, BTU/lbm.

  6. Feb 2, 2014 #5
    I'm not seeing how will I get the units in BTU/lbm.
  7. Feb 2, 2014 #6
    p=14.7 psi = 14.7x144 lbf/ft2
    v=1/62.4 ft3/lbm

    pv = units of ft-lbf/lbm

    What is the conversion factor between ft-lbf and BTU?

  8. Feb 2, 2014 #7
    1.285x10-3 :p thank you for your patience and time.

    Best Regards
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