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Homework Help: Thermodynamic pressure/temperature question

  1. Feb 23, 2009 #1
    1. The problem statement, all variables and given/known data
    Consider filling a cylinder of compressed argon from a high-pressure supply line. Before filling, the cylinder contains 10 bar of argon at room temperature the valve is then opened, exposing the tank to a 50 bar line at room temperature until the cylinder reaches 50bar. The valve is then closed. For argon take C_p =5/2 *R and the molecular weight to be 40kg/mol. You may use the ideal gas model.
    a) what is the temperature after the valve is closed.
    b)if the cylinder sits in storage for a long time how much heat is transferred.
    c) what is the pressure of the cylinder when it is shipped (after sitting a long time)

    2. Relevant equations

    3. The attempt at a solution
    Ok, so I'm not really worried about b or c yet since I can't get a.

    Anyway My attempt:

    I used an equation my prof gave us T2=(P2*(Cp/Cv)*T1)/(P2-P1 + P1/T1*(Cp/Cv)*T1

    I plugged in the given value for Cp used Cv=Cp-R
    used P2=50bar P1=10bar T1=298K and got 59.4 However I am pretty sure the temperature wouldn't go down, if anything it should go up since it is being compressed to a higher pressure. can anyone help maybe this equation is wrong or something? my prof writes really sloppy and I can't read it properly sometimes, let me know if this is the right forum too I wasn't sure where to put this question!
  2. jcsd
  3. Feb 23, 2009 #2

    Andrew Mason

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    Homework Helper

    Think of the gas in the cylinder being compressed by the inflowing gas (eg. think of the argon in the cylinder being contained in a sealed rubber balloon that is then compressed to 50 bar). The inflowing gas, however, does not experience a net compression.

    Work out the temperature for an adiabatic compression of the gas that is originally in the cylinder. Then let the two mix so that the temperature evens out.

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