The enthalpy of steam is determined from the specified pressure and thermodynamic variable using the IAPWS-IF97 steam properties model. The feedwater enthalpy is determined at atmospheric pressure (0.101325 MPaa) and the given feedwater temperature. The difference drives all subsequent energy calculations.

\begin{equation}\label{eq:steam-reduction-enthalpy} \Delta h = h_{steam}(P_{steam}, \theta_{steam}) - h_{feed}(P_{atm}, T_{feed})\end{equation}

Symbols

\(\Delta h\)	Enthalpy difference between steam and feedwater \([\unit{ \kJ\per\kg}]\)
\(h_{steam}\)	Specific enthalpy of steam at operating conditions \([\unit{ \kJ\per\kg}]\)
\(P_{steam}\)	Steam pressure \([\unit{ \mega\pascal}]\)
\(\theta_{steam}\)	Steam thermodynamic variable (temperature, enthalpy, entropy, or quality) \([\unit{ \unitless}]\)
\(h_{feed}\)	Specific enthalpy of feedwater at atmospheric pressure \([\unit{ \kJ\per\kg}]\)
\(P_{atm}\)	Atmospheric reference pressure (0.101325) \([\unit{ \mega\pascal}]\)
\(T_{feed}\)	Feedwater temperature \([\unit{ \K}]\)