Joule Thomson Effect | Joule Thomson Coefficient

Joule Thomson effect

Before moving to the topic we should first know that Joule Thomson effect, Joule Thompson effect, Joule kelvin effect, Joule Thomson expansion, Joule kelvin expansion are the different names of the same thing.

To understand the Joule Thomson effect, it is very important to first understand Throttling process.

Throttling process: Throttling is an irreversible adiabatic process in which a gas is expended by a valve. It is an isenthalpic (constant enthalpy) process.

After throttling a gas becomes either cool or hot, this is known as Joule Thomson effect.

It is interesting to know that why there are two possibilities for temperature (rise or fall). It is due to two factors.

1. In throttling process due to friction (provided by the valve) the pressure energy of the fluid converts into inter-molecular energy. Which results in rise of temperature.
2. In throttling process the fluid expends which results in the drop of temperature.

Hence the rise or fall of the temperature of the fluid after throttling, depends on the dominating factor (from the above two).

If factor (1) dominates the overall effect of throttling will be heating of fluid.

If factor (2) dominates the overall effect of throttling will be cooling of fluid.

Now we can move further in our discussion to understand Joule Thomson effect and Joule Thomson coefficient.

Let’s imagine a throttling process.

Where:

P: Pressure

T: Temperature

h: Enthalpy

For a throttling process we know that

P₁ > P₁’

Now can draw above mentioned throttling process in the T-P graph.

Where:

T _Max: Maximum inversion temperature

T _Min: Minimum inversion temperature

μ_J: Joule Thomson coefficient

Above graph shows different isenthalpic (throttling) curves. Each curve has its own maxima.

By observing the above isenthalpic curves, we can easily say that throttling will only result in cooling when the slope of the curve is positive.

And if the initial temperature of the fluid (used in throttling) is above maximum inversion temperature, then we can never get cooling.

To have maximum cooling the initial state of fluid should lie at inversion curve.

The locus of maxima of these isenthalpic curves is known as inversion curve.

Joule Thomson coefficient

The slope of isenthalpic (throttling) T- P curves is known as Joule Thomson coefficient.

Here one should know that Joule Thomson coefficient, Joule Thompson coefficient, Joule Kelvin coefficient are the different names of the same thing.

Joule Thomson coefficient is represented as μ_J.

μ_J = (∂T/∂P)_h

For ideal gases

μ_J = (∂T/∂P)_h = 0

It means we can never heat or cool ideal gas by throttling.