Expanding a gas through a valve or through a turbine !

  1. Can somebody explain to me why the temperature gets colder when expanding through a turbine versus expanding through a valve.

    Please explain with equations.
  2. jcsd
  3. somebody must know this...
  4. Nugatory

    Staff: Mentor

    Hot gas under pressure stores some potential energy, rather like a compressed spring. This energy is released when the gas expands. If you let the gas out through a valve, almost all of the potential energy goes to speeding up the escaping stream of gas, and this ends up as random kinetic energy of the gas molecules as they bounce around. If the gas expands through a turbine, some of the potential energy ends up as kinetic energy in the rotating turbine, so less ends up as random kinetic energy in the gas molecules.

    Less random kinetic energy in the same (after expansion) volume of gas means a lower temperature.
  5. thank u Nugatory:)
    But I think somebody once explained to me somethng about dQ=dU+dW.
    And that when we expand throgh a turbine, work(dW) is done and therefore we get a much colder gas instead of just expand through a valve.
    Is this just ********??? :):)
    If there is some logic in this, can someone please explain to me what dQ,dU and dW means??
  6. SteamKing

    SteamKing 10,371
    Staff Emeritus
    Science Advisor
    Homework Helper

    dQ is the change in heat energy or enthalpy of a substance
    dU is the change in internal energy of a substance
    dW is the change in energy due to the work of a substance.

    It's not BS. It's the First Law of Thermodynamics.
Know someone interested in this topic? Share this thead via email, Google+, Twitter, or Facebook

Have something to add?