An isenthalpic process is a thermodynamic process in which the enthalpy of a system remains constant. During this process, other properties, such as temperature, pressure, and volume, may change. 

Isenthalpic processes are widely used in systems where fluids undergo pressure reduction with minimal heat transfer and no shaft work. For example, a refrigerator uses this idea when the refrigerant passes through an expansion valve and its pressure drops. ^[1–4]

Formula

Enthalpy (H) is a thermodynamic property defined as the sum of internal energy (U) and the pressure (P) times the volume (V): ^[1,2]

 H = U + PV

For a constant-pressure process with only pressure-volume work, the change in enthalpy equals the heat transferred:

ΔH = ΔQ = ΔU + PΔV

For an isenthalpic process, the enthalpy at the initial state is equal to the enthalpy at the final state:

H₁ = H₂

Therefore,

ΔH = H₂ – H₁ = 0

Here, H₁ and H₂ represent the system’s enthalpy at the initial and final states, respectively.

Because enthalpy is a state function, its change depends only on the initial and final states, not on the path taken between them. Therefore, a process is isenthalpic when the initial and final states have the same enthalpy.

Example

The most common real-world example of an isenthalpic process is throttling. Throttling occurs when a fluid passes through a narrow restriction, such as a valve, capillary tube, or porous plug. As the fluid flows through the restriction, its pressure decreases significantly. ^[2,3]

An ideal throttling process is analyzed using the following assumptions:

• The device is adiabatic, so heat transfer is negligible.
• Shaft work is zero.
• Changes in kinetic and potential energy are negligible.

Throttling is usually described in terms of specific enthalpy, defined as the enthalpy per unit mass. Under the above conditions and using the energy balance equation, the specific enthalpy of the fluid remains nearly constant across the restriction:

h₁ ≅ h₂

Here, h₁ and h₂ represent the specific enthalpy of the fluid before and after throttling. 

The concept of throttling can be extended to ideal and real gases. However, the temperature behavior is different between the two cases. In real gases, this temperature change is explained by the Joule–Thomson effect.

Isenthalpic Processes in Ideal and Real Gases

Whether a real gas cools or warms depends on the type of gas and its starting temperature and pressure. ^[5]

Applications

Isenthalpic behavior is important in several pressure-control and cooling devices: ^[6]

• Throttling valves regulate fluid flow in pipelines and process equipment.
• Pressure regulators maintain a nearly constant outlet pressure in gas supply systems.
• Refrigeration and air-conditioning expansion valves reduce refrigerant pressure before evaporation, allowing heat absorption in the cooling space.
• Gas liquefaction systems use repeated expansion steps to lower the gas temperature until condensation occurs.
• Cryogenic systems use the same principle to obtain very low temperatures for handling liquefied gases.

Isenthalpic processes simplify the analysis of thermodynamic devices in which pressure changes occur without major energy transfer as heat or work.