Joule Thompson effect concepts

These are difficult concepts if you read a book or refer to the internet. This article is written in simple words for easy understanding

Isenthalpic process

It is unique in the sense that despite being an adiabatic process there is no work transfer. H = U + W. Since there is no W transfer the enthalpy H remains the same.dH = 0. In adiabatic processes like adiabatic compression in compressors and adiabatic expansion in turbines, dH is not zero because there is work transfer to surroundings.

In compressors, the interstage isothermal coolers act as work sink. In turbines, a condenser is a work sink. In these processes dQ = 0.

Isentropic process

Unlike JT which is a thermodynamic closed system, the compressors and turbines are open thermodynamic processes. In closed thermodynamic process work W = PdV looks like dominate. In open thermodynamic processes like turbines and compressors W = VdP seems to be dominating.

At constant volume, VdP work which seems to be dominating in compressors and turbines is idealised to be an isentropic process while in JT with PdV work dominating it looks like there is entropy generation internally and hence it is not internally reversible. JT is not isentropic process.

JT coefficient

JT coefficient mu JT is a ratio of T1 - T2 / P1 - P2 across a real gas throttling point in an adiabatic process. For ideal gases delta T / delta P = 0. For real gases when mu JT = 0 there is no temperature change. When mu JT < 0 there is an increase in temperature. When mu JT > 0, there is an increase in temperature.

Inversion temperature

When a real gas expands if there is throttling in the flow of the gas, the point or the temperature at which mu JT = 0 is known as inversion temperature. At inversion temperature, an expanding real gas changes the sign of mu JT. mu JT becomes negative from positive. Positive mu JT signifies expansion and negative mu JT signifies compression of the real gas.

A further expansion beyond inversion temperature makes mu JT < 0. At inversion temperature, a real gas behaves as an ideal gas. At inversion temperature, there is no cooling or heating of a real gas.

In simple words: What happens?

In simple words when a real gas expands it does so by overcoming the intermolecular attraction. The energy comes from internal energy. The gas cools. At inversion temperature, there is no intermolecular attraction or repulsion. The real gas behaves as an ideal gas at an inversion temperature.

Beyond the inversion point, further expansion generates compression because the gas has to push itself to expand by doing thermodynamic work on itself. This increases the internal energy of the gas. The temperature of gas increases.

At inversion temperature, there is no intermolecular expansion or repulsion of gas molecules

When mu JT > 0 intermolecular attractive forces dominate. When mu JT < 0 intermolecular repulsive forces dominate.