Approximate functions for the fast calculation of light-water properties at saturation

For thermalhydraulic systems analysis, linear interpolation algorithms are commonly used for the calculation of thermodynamic properties. However, these algorithms can use a substantial amount of computer time and memory. An alternative to this approach suggested in the past is the use of approximation formulas. Such formulas for the calculation of the thermodynamic properties of light water for saturation conditions are presented here, based on the 1984 NBS/NRC Steam Tables. The range of these approximations is from below lb to just below the critical point (22.055 MPa) with a deviation from tabulated values of not more than 0.22%. The formulas were determined by the method of least squares, enabling a minimization of deviations from the line of best fit and the fitting of functions simple enough to be used with programmable calculators, as well as microcomputers. In addition to the rapid calculation of the properties, the simple curve fits are instrumental in the development of the rate form of the equation of state.Since any given property cannot be accurately fitted over the entire pressure range with a single simple expression, the pressure range was split into subranges. Special care was taken to ensure that the slopes of the curve fits were continuous across the boundaries since discontinuities in the slopes of the property tables can cause instabilities and failure of search algorithms in typical computer codes.