Joule-Thomson Cooling
Expanding a real gas through a restriction cools it below its inversion temperature, a principle used in the final liquefaction stages of cryogenic systems.
Cooling by expansion
When a real gas expands through a valve or porous plug from high to low pressure without doing external work or exchanging heat, its temperature changes. This is the Joule-Thomson effect. Whether the gas cools or warms depends on its temperature relative to its inversion temperature: below the inversion temperature it cools, above it warms. The effect is a workhorse of practical refrigeration and gas liquefaction.
Why real gases behave this way
An ideal gas would show no temperature change on such expansion, because its internal energy depends only on temperature. Real gases have intermolecular forces. At lower temperatures the attractive forces dominate, so pulling molecules apart during expansion costs energy drawn from their thermal motion, cooling the gas. At high temperatures repulsive interactions dominate and expansion warms the gas. The crossover is the inversion temperature, characteristic of each gas.
The helium complication
Helium has a very low inversion temperature, around 40 kelvin. This means helium gas at room temperature actually warms on Joule-Thomson expansion. To liquefy helium, it must first be precooled below its inversion temperature by other means, such as expansion engines or a bath of colder cryogen, before a final Joule-Thomson stage can produce liquid. This is why helium liquefiers combine expansion turbines or pistons with a final throttle.
Where it appears in cryogenics
Joule-Thomson stages appear in helium liquefiers, in some closed-cycle coolers, and historically in the condensing lines of wet cryostats. The principle is elementary but its correct application, respecting the inversion temperature, is essential to any system that condenses helium, including the pathways that feed the mixtures used in dilution refrigerators.
- Real gas expanding through a restriction cools if below inversion temperature
- Ideal gases show no Joule-Thomson temperature change
- Helium's inversion temperature is low, around 40 K
- Helium must be precooled before a Joule-Thomson stage liquefies it