# Expanding a gas through a valve or through a turbine !

1. Dec 27, 2012

### charlie95

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. Dec 28, 2012

### charlie95

somebody must know this...

3. Dec 28, 2012

### 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.

4. Dec 28, 2012

### charlie95

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??

5. Dec 28, 2012

### SteamKing

Staff Emeritus
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.

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