TPWS

TPWS

Thermodynamic Properties of Water and Steam

by Howard Chao and Stella Unruh

Introduction

Properties of water and steam are vital to the successful design and operation of most industrial plants. The calculation of the thermodynamic properties of water can be tedious and time consuming through the use of steam tables and diagrams. Furthermore, obtaining highly accurate and precise data by these methods may be difficult if not impossible. Therefore the need for computer programs capable of finding this data quickly and accurately is great.

The objective of this project was to create a robust program which provides values for various thermodynamic properties of water and steam at a given temperature and pressure with a built in unit converter. In addition to providing an electronic steam table, these calculated properties are continently stored in data structures for additional analysis and processing. With these main features in place, the code may be easily adapted and modified to suit advanced processes and calculations.

The thermodynamic properties of water and steam were calculated using the most recent standards (IF-97) set forth by The International Association for the Properties of Water and Steam (IAPWS). The IAPWS IF-97 formulation covers the following range of validity

273.15 K ≤ T ≤ 1073.15 K p ≤ 100 MPa

1073.15 K ≤ T ≤ 2273.15 K p ≤ 10 MPa.

The figure below depicts the five distinct regions which are covered by the IAPWS IF-97 on the phase diagram for water.

For each region a different generating function is used to calculate the properties for points within that area. Regions 1, 2 and 5 are specifically covered by the specific Gibbs free energy (as a function of temperature and pressure) while Region 3 is represented by the specific Helmholtz free energy (as a function of temperature and density). For the purposes of our project, iterations must be performed to determine the density of water corresponding to a given pressure and temperature point in Region 3. Region 4 corresponds to the liquid/vapor coexistence curve and culminates at the critical point (647.096 K, 22.064 MPa). From the expressions for the free energies, all of the relevant thermodynamic properties may be easily evaluated by taking the appropriate partial derivatives.