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Homework Help: Thermodynamics - Find initial pressure (given Temperatures)

  1. Sep 25, 2010 #1
    1. The problem statement, all variables and given/known data

    For a certain experiment, R-410a vapor is contained in a sealed glass tube at 20°C. It is
    desired to know the pressure at this condition, but there is no means of measuring it, since
    the tube is sealed. However, if the tube is cooled to −20°C small droplets of liquid are
    observed on the glass walls. What is the initial pressure?

    2. Relevant equations

    Thermodynamic tables
    ideal gas equation?

    3. The attempt at a solution

    I set up both states:
    State 2: T = -20 (saturated)

    State 1: T = 20, v1=v2=0.0648 P = ? (superheated)

    I just don't understand how we can figure it out from just the temps. I just cant wrap my mind around the thermodynamic tables.
     
  2. jcsd
  3. Sep 26, 2010 #2

    Borek

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    Staff: Mentor

    What happens at -20?
     
  4. Sep 26, 2010 #3
    At -20, the condensation occurs?
     
  5. Sep 26, 2010 #4

    Borek

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    Staff: Mentor

    Can you use it to find vapor pressure at -20? With the help of tables?
     
    Last edited by a moderator: Aug 13, 2013
  6. Sep 26, 2010 #5
    Is it read directly from the table? would it be 399.6 kPa?
     
  7. Sep 26, 2010 #6

    Borek

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    Staff: Mentor

    No idea, I don't have these tables. But yes, you should read it directly from the table.
     
    Last edited by a moderator: Aug 13, 2013
  8. Sep 26, 2010 #7
    So, how would I use this information to find the initial pressure?

    Could I just use the ideal gas? P1/T1 = P2/T2 ?
     
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