All courses › Thermodynamics › Throttling and isentropic processes

Throttling and isentropic processes

Throttling is an irreversible process where a gas or liquid is forced through a narrow opening, such as a valve, with no work done and no heat exchanged. The enthalpy stays approximately constant, but the pressure and usually the temperature drop. An isentropic process, by contrast, is reversible and adiabatic, and is used to idealize turbines and compressors.

h1≈h2h_1 \approx h_2throttling, approximately constant enthalpy
Q=0, ΔS=0Q = 0,\ \Delta S = 0isentropic process

Symbols

hhspecific enthalpykJ/kg
SSentropyJ/K

Example

Refrigerant is throttled through an expansion valve. The enthalpy before and after is roughly equal, but the pressure drops sharply, and part of the liquid flashes to vapor and cools the rest.

An isentropic process is a loss-free idealization – real turbines and compressors come close to it, but never exactly reach it.
Practise thermodynamic cycles for free →

← Idealized thermodynamic cycles · Coefficient of performance (COP) →

Part of Thermodynamics: Thermodynamic cycles.