Isenthalpic process

An isenthalpic process is one that proceeds without any change in enthalpy, H; or specific enthalpy, h.

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In an isenthalpic process there is no transfer of heat to (or from) the surroundings, and no work done on (or by) the surroundings. There will usually be significant changes in pressure and temperature during the process.

The throttling process is a good example of an isenthalpic process. Consider the lifting of a relief valve or safety valve on a pressure vessel. The specific enthalpy of the fluid inside the pressure vessel is the same as the specific enthalpy of the fluid as it escapes from the valve. With a knowledge of the specific enthalpy of the fluid, and the pressure outside the pressure vessel, it is possible to determine the temperature and speed of the escaping fluid.

Another example of an isenthalpic process is an adiabatic mixing bath. Hot water at pressure A mixes with cold water at pressure B and flows out at pressure C. There is no work done on the surroundings; nor is there heat transfer. Thus, enthalpy is preserved.

In an isenthalpic process:

• h₁ = h₂
• dh = 0

Isenthalpic processes on an ideal gas follow isotherms since dh = C_p0(T₂ − T₁)

See also

• Isentropic process
• Adiabatic process