Details

Water at high pressure undergoes Joule–Thomson expansion when passing through a throttling valve. This process is assumed to be adiabatic, so the enthalpy change is zero.

, so , where is the inlet enthalpy and is the exit enthalpy.

When two phases are present, the saturated vapor enthalpy and the saturated liquid enthalpy are used to determine the fraction of the exit stream that is vapor. For the conditions chosen, liquid and vapor exit the throttle at saturation conditions, because .

The steam quality is calculated by:

,

so that .

When only liquid is present and .

The screencast video at [1] explains how to use this Demonstration.

Reference

[1] Throttling High-Pressure Water. www.colorado.edu/learncheme/thermodynamics/ThrottlingWater.html.