Joule–Thomson effect

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Joule-Thomson sign

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Joule-Thomson curves 2

Dieterici gas inversion temperature plot

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Throttling in Ts diagram 01

Joule–Thomson Effect (also known as the Joule-Kelvin effect) is a thermodynamic process that describes the temperature change of a real gas or liquid when it is forced through a valve or porous plug while keeping it insulated so that no heat is exchanged with the environment. This phenomenon is named after James Prescott Joule and William Thomson, 1st Baron Kelvin, who discovered it in 1852.

Overview

The Joule–Thomson effect is significant in the field of thermodynamics, particularly in processes involving the liquefaction of gases. When a gas expands, its pressure decreases, and it does work on its surroundings. If this expansion is adiabatic (no heat is exchanged with the surroundings), the energy to do this work comes from the internal energy of the gas, which results in a change in temperature. The direction of the temperature change depends on the specific characteristics of the gas and the initial temperature and pressure.

Mechanism

The mechanism behind the Joule–Thomson effect can be explained by the real gas behavior, which deviates from the ideal gas law. Real gases have intermolecular forces, and their molecules occupy a finite volume. During the Joule–Thomson expansion, the decrease in pressure allows molecules to move farther apart, which can either absorb or release energy depending on the nature of the intermolecular forces. For most gases at room temperature, this results in a cooling effect as the gas expands, but under certain conditions, it can also lead to heating.

Applications

The Joule–Thomson effect has practical applications in various industries. It is the principle behind refrigeration and air conditioning systems, where gases are compressed and expanded to cool the air. It is also used in the liquefaction of gases, such as oxygen, nitrogen, and natural gas, which are important in medical, industrial, and energy sectors.

Joule–Thomson Coefficient

The Joule–Thomson coefficient is a measure of the temperature change per unit pressure change during a Joule–Thomson expansion. It is positive for gases that cool upon expansion (under certain conditions) and negative for those that heat up. The coefficient varies with temperature and pressure, and its calculation is crucial for designing equipment that utilizes the Joule–Thomson effect.

Limitations

The Joule–Thomson effect is limited by the inversion temperature, above which the effect reverses, and the gas warms instead of cooling upon expansion. This limitation is significant in the design of refrigeration and gas liquefaction processes, where the working substance must be chosen and the process conditions carefully controlled to achieve the desired temperature change.

See Also

• Thermodynamics
• Ideal gas law
• Real gas
• Liquefaction of gases
• Refrigeration
• Inversion temperature