Expanding a gas through a valve or through a turbine

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Discussion Overview

The discussion revolves around the thermodynamic behavior of a gas when it expands through a turbine compared to a valve. Participants explore the implications of these different expansion methods on temperature changes, with references to thermodynamic principles and equations.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant questions why the temperature decreases when a gas expands through a turbine rather than through a valve, seeking an explanation supported by equations.
  • Another participant suggests that when gas expands through a valve, most potential energy converts to kinetic energy of the gas stream, resulting in increased random kinetic energy and thus higher temperature.
  • In contrast, expansion through a turbine allows some potential energy to be converted into work done on the turbine, leading to less random kinetic energy in the gas and a lower temperature.
  • A participant references the First Law of Thermodynamics, mentioning the relationship between heat energy (dQ), internal energy (dU), and work (dW) during the expansion process.
  • There is a request for clarification on the meanings of dQ, dU, and dW, indicating a desire for deeper understanding of the thermodynamic concepts involved.

Areas of Agreement / Disagreement

Participants express differing views on the mechanisms behind temperature changes during gas expansion, with some supporting the turbine explanation while others seek further clarification. The discussion remains unresolved regarding the complete understanding of the thermodynamic principles at play.

Contextual Notes

Participants reference the First Law of Thermodynamics but do not fully resolve the implications of dQ, dU, and dW in the context of their discussion. The specific conditions under which these expansions occur are not detailed.

charlie95
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Can somebody explain to me why the temperature gets colder when expanding through a turbine versus expanding through a valve.

Please explain with equations.
 
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somebody must know this...
 
charlie95 said:
Can somebody explain to me why the temperature gets colder when expanding through a turbine versus expanding through a valve.

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.
 
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??
 
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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