APPROXIMATE RELATIONSHIPS FOR DETERMINING THE ENTHALPIES OF WATER AND STEAM IN THE CALCULATIONS OF HEAT EXCHANGER DEVICES OF STEAM TURBINE INSTALLATIONS (PART I – DETERMINATION OF THE ENTHALPY OF WATER)

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The work is devoted to the creation of a methodological approach for determining the thermodynamic properties of water and water vapor in the calculation of heat exchange processes for those cases when it is necessary to quickly control the operating modes of steam turbines of large power of thermal and nuclear power plants, but the use of existing tables of properties of working environments in the "manual" mode is not always suppose. In the presented part of the work the determination of the enthalpy of water is considered. Based on the analysis of the operating modes of heat exchangedevices of power units thermal and nuclear power plants of different capacities, the ranges of temperature and pressure changes of the working media during their calculations were determined. They are for water: at a pressure of 1 kPa–30 MPa, at a temperature of 1–300 °С, and for steam, respectively: 1 kPa–6 MPa, 7–450 °С. The analysis of tabular values of the working medium (water) showed that in order to present the enthalpy of water in an analytical form based on approximation equations with the required accuracy, it is advisable to consider their construction in two areas of change: in the area of vacuum (Р < 0.1 MPa) and in the area of excess pressure (0.1–30 MPa), which in turn are divided into 4 ranges. When thepressure changes, this is associated with an additional effect on the temperature enthalpy value, which complicates the choice of approximate dependencies for obtaining enthalpy from the parameters of the working environment while ensuring an acceptable ratio of calculated and tabular values for solving problems. By the authors a system of regression equations was obtained, allowing to calculate the enthalpy of water in a given range of pressures and temperatures with high accuracy. To determine the area in which the measured value is located, the authors proposed the use of Antoine's formula, in which the relationship between the pressure and temperature of the medium at the boundary line of the phase transition is established. To increase its accuracy, approximation equations for temperature corrections at different pressures are proposed, which ensure that the temperature deviates from the tabulated values of thermophysical properties there are no more than ±0.04 °С.