Isoenthalpic means constant enthalpy, and any material which passes through a system without a change of enthalpy has, by definition, passed through an isoenthalpic process. For an open system at steady flow:
If, in addition, the process is adiabatic (Q = 0) and there is no work transfer (W = 0), then if the sum of the changes in kinetic and potential energy are zero, the process is isoenthalpic. For instance, it is common to assume that a gas or vapor flashing through a valve is an isoenthalpic process. In such an example, the residence time and contact area available within the valve is so small that very little heat transfer can occur, so the process is approximately adiabatic. The valve does not transfer work to the surroundings and the inlet and outlet are at such similar elevations that changes of gravitational potential energy can be neglected. Finally, it is common to neglect the change in kinetic energy since the diameters of the inlet and outlet pipes can be, and often are, selected to minimize the change in velocity of the fluid.
The assumption that a process is isoenthalpic gives a simple method for determining the change in temperature of fluid flowing through the process, provided the upstream conditions and the downstream pressure are known, as follows:
Knowing the upstream conditions, the upstream enthalpy is known or can be calculated. Knowing that the downstream enthalpy is the same as the upstream enthalpy and knowing the downstream pressure, the temperature and condition of the downstream fluid can be determined.