**ES_StableIF97
AddIn**

**
Metastable Vapor Functions**

IAPWS-IF97 provides the statements about properties in each metastable regions as below.

Region 1 : The basic equation of region 1 (Equation 7 of IAPWS-IF97) yields reasonable values in the metastable superheated-liquid region close to the saturated liquid line.

Region 2 : The basic equation of region 2 (Equation 15 of IAPWS-IF97) yields reasonable values in the metastable-vapor region for pressures above 10 MPa. The basic equation is not valid in the metastable-vapor region at pressures p <= 10 MPa; for this part of the metastable-vapor region IAPWS-IF97 provides a supplementary equation (Equation 18 of IAPWS-IF97). IAPWS-IF97 states that the supplementary equation is valid in the metastable-vapor region from the saturated vapor line to the 5% equilibrium moisture line (determined from the equilibrium enthalpies h' and h" values) at pressures from the triple-point pressure up to 10 MPa. If h' = 100 and h" = 600, the 5% equilibrium moisture line is calculated as (600 - 100) * 0.95 + 100 = 575. Please note that the triple point is 273.16 K temperature and 611.657 Pa pressure.

Region 3 : The basic equation of region 3(Equation 28 of IAPWS-IF97) yields reasonable values in the metastable regions (supreheated liquid and subcooled steam) close to the saturated liquid and saturated vapor line for T >= 623.15 K to Tc(= 647.096 K). Tc is critical temperature.

The IAPWS-IF97's statement above means that for metastable state the basic equations for Region 1, Region 3 and 10 MPa pressure and above for Region 2, can be used as it is. However, for 10 MPa pressure and below of Region 2 a separate equation is required to calculate metastable vapor state.

In this concern, ES_StableIF97 AddIn provides metastable-vapor properties in region 2 in accordance with the supplementary equation of IAPWS (Equation 18 of IAPWS-IF97). The metastable-vapor properties in region 2 are normally used in industries.

1) Valid Range of Metastable-Vapor Properties : 0.00611213 bar <= Pressure <= 100 bara, 273.15 K <= Temperature <= 1073.15 K

2) Properties are calculated, given pressure and temperature. Properties calculated are specific enthalpy(h), specific entropy(s), specific volume(v), specific internal energy(u), specific isobaric heat capacity(Cp), specific isochoric heat capacity(Cv) and speed of sound(w).

3) If the pressure and temperature given is saturation state, saturation vapor properties are returned. The saturation vapor properties are calculated by the basic equations instead of metastable equations.

4) If the pressure and temperature given is superheated vapor state, superheated vapor properties are returned. The superheated vapor properties are calculated by the basic equations of Region 2.

5) The metastable vapor properties returned should be checked for validity by user, i.e. within 5% equilibrium moisture line calculated by enthalpy. The metastable vapor functions of ES_StableIF97 AddIn do not return error message, but do return the property calculated by the metastable equation.

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