Steam Quality from Enthalpy

Method & assumptions

1. Pure water at liquid–vapor equilibrium, using absolute pressure in Pa. The range is 611.657 Pa through pressures strictly below 22064000 Pa. Quality is undefined at the critical point; numerically unresolved phase boundaries are rejected.
2. Saturation temperature and separate liquid/vapor properties use IAPWS SR1-86(1992). The release’s rounded lower pressure endpoint maps to the triple-point temperature. Ice and dissolved substances are excluded.
3. Quality x is vapor mass divided by total mixture mass. Enter a fraction, such as 0.9 for 90%. Specific volume, enthalpy and entropy are mass-weighted: y = (1 − x)y_liquid + x y_vapor. Density is 1/v, not a mass-weighted average of the two densities.
4. Vapor volume fraction = x v_vapor / v_mixture. This describes a saturated inventory. It is not a flowing-pipe void-fraction prediction from vapor mass flow fraction; phase slip and flow patterns are not modeled.
5. Mixture internal energy u = h − pv. Enthalpy and entropy retain the IAPWS reference convention. For the inverse calculation, x = (h − h_liquid)/(h_vapor − h_liquid); values outside the model’s saturated enthalpy interval are rejected rather than clamped. Near-critical inversion is sensitive as the enthalpy gap shrinks.

Sources

• IAPWS SR1-86(1992): Saturation phase properties ↗
• DOE Thermodynamics Volume 1: quality, mixture properties and enthalpy definitions, pages 18, 34 and 49 ↗