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Mechanical Engineering Placement Thermodynamics Problem

  1. Mar 14, 2012 #1
    Hi all I am struggling with this classs example

    A pump compressing hydrogen from a low pressure to a high pressure tank where the tanks pressure is constant and H2 is an ideal gas.
    Hydrogen: (H2: MW = 2, Cv = 10 kJ/kgK)
    Low pressure tank: (P = 4 bar, T = 30oC)
    High pressure tank: (P = 100 bar).

    1) Apply the first law of thermodynamics eliminating terms not needed
    2) Find the compressor outlet temp when the compression goes along an adiabatic and polytrophic process
  2. jcsd
  3. Mar 14, 2012 #2


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    Since you stated this is a homework problem, what formulas are you using and what have you attempted so far?
  4. Mar 14, 2012 #3

    For the first law I have used Q-W = difference in energy and attempted to find the temp via finding the internal energy and enthalpy using deltau = cv x delta T and delta h = cp x delta T where I got 8520 KJ and 12063 KJ but now I am not sure how if I should integrate and remove pressure from the first law and find the temp now.
  5. Mar 14, 2012 #4


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    The problem states that Hydrogen can be considered an ideal gas, so it seems to me that all you really need to do is apply the ideal gas law using pressure and temperature at the input, and pressure at the output. Adiabatic means there is no heat entering or leaving the system during the process, so the gas is not cooled or heated by any other process.

    As for the polytropic process, that will take a bit of fitting work to figure out what the polytropic index and constant is. Are you familiar with a polytropic process's form?
  6. Mar 14, 2012 #5
    firstly for the first law does one simply remove pressure from the closed loop first law of thermodynamics?
  7. Mar 14, 2012 #6


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