An isenthalpic process or isoenthalpic process is a process that proceeds without any change in enthalpy, H; or specific enthalpy, h.
In a steady-state, steady-flow process, significant changes in pressure and temperature can occur to the fluid and yet the process will be isenthalpic if there is no transfer of heat to or from the surroundings, no work done on or by the surroundings, and no change in the kinetic energy of the fluid. (If a steady-state, steady-flow process is analysed using a control volume everything outside the control volume is considered to be the surroundings.)
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.
In an isenthalpic process:
Isenthalpic processes on an ideal gas follow isotherms since .
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