The Joule-Thomson (JT) effect is a thermodynamic process that occurs when a fluid expands from high pressure to low pressure at constant enthalpy (an isenthalpic process). Such a process can be approximated in the real world by expanding a fluid from high pressure to low pressure across a valve. Under the right conditions, this can cause cooling of the fluid.
This effect was first observed in an experiment conducted by James Joule and Thomson in 1852 in which they flowed high pressure air through a small porous plug causing the pressure to drop. Joule and Thomson noted that the air was cooled by this procedure (which is a good approximation of an isenthalpic process).
Building on this work, in 1895, Linde in Germany and Hampson in England independently developed and patented a refrigerator that combined the JT effect with heat exchangers and a piston compressor. This became known as the Linde-Hampson or Joule–Thomson cycle. Such a refrigerator played an important role in James Dewar’s liquefaction of hydrogen in 1898.
The Joule-Thomson effect can be described by means of the Joule-Thomson coefficient which is simply the partial derivative of the pressure with respect to temperature at constant enthalpy. If this coefficient is positive, then the fluid cools upon expansion and if it’s negative the fluid warms upon expansion. The JT coefficient varies as a function of pressure and temperature and varies from fluid to fluid. The curve described by the JT coefficient equaling zero as a function of pressure and temperature is known as the inversion curve. Underneath this curve cooling of the fluid occurs upon expansion. It is also possible to define for a given fluid the maximum inversion temperature. The fluid must be colder than this temperature to cool when expanded.
In the case of nitrogen, the maximum inversion temperature is 623K. It is completely possible to make liquid nitrogen simply by using a high pressure cylinder of room temperature nitrogen, a JT valve and appropriate heat exchangers that cool the higher pressure room temperature gas with gas that has expanded through the JT valve and is thus colder.
Joule-Thomson refrigerators do not have cold moving parts and are able to work well with fluids that change from single phase to two-phase as they expand through the JT valve. Such devices may also have miniaturized cold ends and may be designed to rapidly cool down to operating temperature from room temperature. Due to these advantages, Joule-Thomson liquefiers/refrigerators have been used for a variety of applications in the aerospace, defense and medical fields.
While they have to some extent been replaced by pulse tube cryocoolers, JT liquefiers and refrigerators remain an important class of small cryocooler either alone or in combination with other cryocooler types.
The maximum inversion temperature of helium is 43K, meaning that helium has to be cooled to this temperature before the JT effect will cause cooling rather than warming. As a result, there is generally a JT valve near the lowest temperature portion of helium refrigerators/liquefiers to provide the final cooling stage. Expansion through a JT valve is also the final step in producing He II (superfluid helium, T < 2.2K) from liquid helium at 4.2K. It’s worth noting that the inversion curve for helium between 1.2K and 2.2K is actually negative, meaning that in most of the He II regime itself, isenthalpic expansion of the helium causes a slight amount of heating.
Reference - cryogenicsociety