isenthalpic expansion is through PRDS or control valve where entropy changes. Whereas expansion through a steam turbine is isentropic one and enthalpy drops. isentropic expansion is more efficient process as compared to isenthalic one.

Practically there is no reversible isentropic process but to make the concept easier to be understood, you have to assume the following:

* Ideal gas.

* no friction losses.

* Adiabatic preocess (no heat gain, no heat loss).

API 520 part 1 Appendix B assumes that the vapor expansion through a nozzle or a pressure relief valve follows an isentropic path.

R. Edse has written:

'Design of supersonic expansion nozzles and calculation of isentropic exponent for chemically reacting gases'

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Isentropic materials are materials that undergo a reversible, adiabatic process where there is no change in entropy. This means that the material experiences no energy transfer as heat, and its entropy remains constant during the process. Isentropic materials are often used in thermodynamic studies and calculations.