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Homework Help: High pressure humid air and thermodynamic tables

  1. Dec 20, 2009 #1
    1. The problem statement, all variables and given/known data
    This is my problem: i have a container with humid (say psi = 60%) air at a pressure of 450 bar. i need to know to what preesure can this air be expanded (adiabatically) so that it reaches temperature just above freezing.
    i also need to know how much work is extracted from this expansion.

    2. Relevant equations
    this would not be hard to calculate using ideal gas law and laws for gas mixtures, but at this pressure the compressibility factor is 1.4, that means a 40% error done with ideal gas law. this should be done using thermodynamic tables and diagrams.

    3. The attempt at a solution
    as air expands adiabatically, it cools down, but condensing water adds some heat so expansion of "the dry part" is actually polytropic. if i neglect partial pressure of water vapor, i can calculate polytropic exponent n, since i know how much heat the water gives out during condensation.
    but i don't know how partial pressure of water vapor changes with pressure.
    if dew point changes with pressure, the complication is even bigger.
    if i knew enthalpies in start and end state, calculating work would be simple (w = |h2 - h1|).
  2. jcsd
  3. Feb 18, 2010 #2
    You need to use a psychrometric chart (http://en.wikipedia.org/wiki/Psychrometrics). Adiabatic cooling is a transition along a constant enthalpy line.

    See also http://www.eitexam.com/Search2/ZMech/PsychroEq.asp?SB=1#Adiabatic%20Mixing [Broken]

    The answer should pop out.
    Last edited by a moderator: May 4, 2017
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