IMPROVED EQUATIONS FOR BACKWARD FUNCTIONS T=f(p,h), T=f(p,s), T_s=f(p) - A CONTRIBUTION TO THE IAPWS PROJECT "NEW INDUSTRIAL FORMULATION"

The paper presents improved equation structures for g=f(T,p), T=f(p,h) and T=f(p,s) in the steam region, equation structures for T=f(p,h) and T=f(p,s) in the liquid region and various equation pairs p_s=f(T) and T_s=f(p) on the saturation curve which are the subject of the current IAPWS project "New Industrial Formulation". The established equations fall within the uncertainty limits set by the IAPWS 1985 Skeleton Tables when compared to the Saul/Wagner equation converted to the new Temperature Scale ITS 90. The numerical consistency of the backward.equations meets the demands of process modeling which were set by the IAPWS Subcommittee on Industrial Calculations. Using these equations in process modeling eliminates the otherwise necessary iterations in calculating backward functions. The structures of the equations have been optimized with the Algorithm of Setzmann and Wagner. After that equations have been made numerically consistent with the simultaneous approximation method developed by Zschunke and advanced by Willkommen.