Joule-Thomson coefficient is defined as the rate of change of temperature with pressure during an isenthalpic process or throttling process.

Mathematically, Joule-Thomson coefficient (μ)can be given as

It is defined in terms of thermodynamic properties and is itself a property.

Joule-Thomson coefficient gives slope of constant enthalpy lines on temperature—pressure diagram.

Thus, it is a parameter for characterizing the throttling process.

Slope of isenthalpic line may be positive, zero or negative, i.e. μ > 0,

μ = 0 and μ < 0 respectively. Mathematically evaluating the consequence of μ we see,

– for μ > 0, temperature decreases during the process.

– for μ = 0, temperature remains constant during the process.

– for μ < 0, temperature increases during the process.

Joule-Thomson expansion can be shown as in Fig.

Here gas or liquid is passed through porous plug for causing isenthalpic process. Valve put near exit is used for regulating pressure after constant enthalpy process i.e. p2.

If pressure p2 is varied then the temperature variation occurs in the isenthalpic manner as shown in T-p diagram. This graphical representation of isenthalpic curve gives the Joule Thomson coefficient by its slope at any point. Slope may be positive, negative or zero at different points on the curve.

The points at which slope has zero value or Joule-Thomson coefficient is zero are called “inversion points” or “inversion states”. Temperature at these inversion states is called “inversion temperature”. Locii of these inversion states is called “inversion line”.

Thus, inversion line as shown divides T-p diagram into two distinct region i.e. one on the left of line and other on the right of line.

For the states lying on left of the inversion line temperature shall decrease during throttling process while for the states on right of inversion line throttling shall cause heating of fluid being throttled.

Temperature at the intersection of inversion line with zero pressure line is called “maximum inversion temperature”.