JT valve works on the principle that when the pressure of a gas/Liquid changes, its temperature also changes.
The Joule–Thomson effect describes the temperature change of a gas or liquid when it is forced through a valve, while kept insulated. ( insulation is required to avoid influence of surrounding environment effects).
Fig: Joule–Thomson coefficients for various gases at atmospheric pressure.
The rate of change of temperature (T) with respect to pressure P is a Joule Thomson process (at constant enthalpy).
μJT = (∂T/∂P) H
μJT = Joule Thomson Coefficient expressed in °C/bar or K/Pa
∂T = Change in temperature
∂P = Change in Pressure.
H= Constant Enthalpy.
All real gases have an inversion point at which the μJT changes sign. The temperature of this point the Joule-Thomson inversion temperature, depends on the pressure of the gas before expansion.
In a gas expansion the pressure decreases below the inversion temperature – the sign ∂P is negative by definition.
With that in mind, if the gas temperature is below the inversion temperature then μJT is positive since ∂P always negative thus ∂T must be negative. So the gas cools.
μJT > 0, then temperature decreases, gas cools.
In a gas expansion the pressure decrease and the gas temperature is above the inversion temperature then μJT is negative, ∂P always negative thus ∂T must be positive. So the gas warms.
μJT < 0, then temperature increase, gas warms.
JT valve is used to avoid formation of hydrates down stream of control valve in winter.