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Joule–Thomson effect
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Temperature change when a gas expands through a valve.
A real gas expanding through a porous plug changes temperature at constant enthalpy; below the inversion temperature it cools. It's used for liquefying gases.
Refrigeration and liquid nitrogen production.
An ideal gas doesn't change temperature.
In more detail
The Joule-Thomson effect is the temperature change of a gas when it expands through a throttle at constant enthalpy. Most gases cool on expansion below an inversion temperature.
Real-world uses
- Refrigerators and air liquefaction use the Joule-Thomson effect.
- Industrial gas plants cool gases to produce liquids.
- Refrigerators use the Joule–Thomson effect to cool gases by letting them expand through a valve.
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Frequently asked questions
Why does the gas cool?
Because it does work against intermolecular attractions as it expands.
Do all gases cool?
Not above the inversion temperature, where they warm instead.
Does every gas cool on expansion?
No. Only gases below their inversion temperature cool when they expand through a valve, while others warm up.
Related topics in Heat & Thermodynamics
Third law · Maxwell relations · Clausius–Clapeyron equation · Heat death of the universe · Arrow of time · Humidity and dew point