What is Joule Thomson Effect in Thermodynamics?

The Joule Thomson effect displays the decrease or increase in temperature of the gas or liquid during an isenthalpic expansion (throttling process) when moving freely through some valve or through other restraining devices while keeping the entire process insulated so that no heat escapes or enters the device. The change in temperature during the process of expansion depends not only upon the pressure but also on the manner of expansion. For instance, if the expansion process is reversible, the gas would be in a state of thermodynamic equilibrium, and the expansion is isentropic. Once you have understood this concept, do practice the numerical section of NCERT class 11 thermodynamics exercise. The problems covered on the Joule-Kelvin effect are perfect for testing what you have learnt.

As per Thermodynamics, when a gas slips through a throttling valve and its pressure drops, the change in temperature depends on one number—the Joule–Thomson coefficient, μJT. In the next five short steps, you’ll see where that coefficient comes from, why it vanishes for an ideal gas, and how it predicts cooling-versus-heating in real gases.

1. Start with the throttling fact

A throttling (Joule–Kelvin) expansion is isenthalpic:

$\Delta H=0$

2. Write the total differential of enthalpy

$$\mathrm{dH}=\left({\frac{\partial H}{\partial T}}_P\right)\mathrm{dT}+\left({\frac{\partial H}{\partial P}}_T\right)\mathrm{dP}=0$$

3. Replace the partial derivatives with measurable terms

${\left(\frac{\partial H}{\partial T}\right)}_P=C_P$ Constant-pressure heat capacity 

A maxwell relation gives ${\left(\frac{\partial H}{\partial P}\right)}_T=V-T{\left(\frac{\partial V}{\partial T}\right)}_P$

​4. Solve for  ${\left(\frac{\partial T}{\partial P}\right)}_H$

Setting 𝑑𝐻 = 0 and rearranging:

$${\mu }_{JT}={\left(\frac{\partial T}{\partial P}\right)}_H=\frac{T\left({\frac{\partial V}{\partial T}}_P\right)-V}{C_P}$$

That fraction is the Joule–Thomson coefficient, μJT.

Following points highlight the properties of Joule-Thomson effect:

1. The Joule-Thomson effect is based on the principle of transfer of heat.
2. For an ideal gas, the Joule-Thomson coefficient is always zero because the enthalpy of the gas is dependent on the temperature. 
3. The Joule-Thomson effect does not affect hydrogen gas in higher temperatures because, at high temperatures, the gas behaves as an ideal gas. So, it affects hydrogen at lower temperatures.

The Joule Thomson effect can be described through the Joule-Thomson coefficient. The formula for the Joule-Thomson effect is μJT = (∂T/∂P)H 

When there is no change in pressure, even when the temperature is decreased, that temperature is known as inversion temperature.

In the chapter, ‘Thermodynamics’, you would get to know about thermal equilibrium, laws of thermodynamics, thermodynamic processes, heat engines, reversible and irreversible processes, along with the Joule-Thomson effect.

Following are the applications of the effect:

1. The Joule-Thomson effect is used in the petrochemical industry in the Linde technique, where the cooling effect is used for liquefying the gases.
2. Cooling produced during Joule Thomson expansion makes it highly useful in refrigeration process.
3. Joule-Thomson cooling effect is also used in various cryogenic processes such as for producing liquid Nitogen, Oxygen and Argon.
4. Joule-thomson effect in thermodynamics is also used for liquifying Helium.

5. Joule Thomson effect delivers reliable, vibration-free cooling wherever a pressure drop is easy to engineer. Knowing its range—cooling below the inversion temperature, warming above—lets designers choose the right gas and throttling stage for every chill-down job.

6. JT valves act like passive coolers along CO₂ and H₂ pipelines. This offsets compression heat.

The inversion temperature, T_inv, is the precise temperature at which the Joule-Thomson coefficient, μ_JT (= ∂T/∂P at constant enthalpy), changes sign.

• Below T_inv: μ_JT > 0 → the gas cools down when throttled.
• Above T_inv: μ_JT < 0 → the gas warms up when throttled.

Knowing a gas’s inversion temperature tells you instantly whether a JT throttling step will give you useful refrigeration or an unwanted temperature rise.

The following image represents the Joule-Thomson temperature inversion curve:

In the above curve:

• • Pr is the reduced pressure which is the actual pressure divided by critical pressure of gas.
  • Tr is the reduced temperature which is the actual temperature divided by the critical temperature.
• On the blue curve,  μJT (JT coefficient) is exactly 0. This means that even on throttling a gas, no temperature change will happen.
• Left of the curve is the cooling region. If a gas is throttled here, it will cool down.
• Right of the curve is the heating region. If a gas is throttled here, it will heat up.