The properties of water and steam play a critical role in the design and performance of steam boilers, steam turbines, and other steam-related equipment. Even small discrepancies in the determination of key properties, such as enthalpy, can result in thousands of dollars lost in turbine or boiler performance. These aspects place great importance on the accuracy of steam tables, which are now mostly superseded by computerized steam and water property software.
During the 1920s and 1930s, engineers and scientists desperately felt the need to agree on an internationally acceptable table, which should be reasonably accurate, for the properties of water and steam that could be legally binding on buyers and suppliers. After much debate and lengthy discussion, the international scientific and engineering community agreed on some initial tables of properties for a limited range of pressure and temperature. These tables were published in 1934 and became a vital tool for engineers and became famous as “steam tables”. Those international discussions gave rise to the organization called the International Association for the Properties of Water and Steam, or IAPWS.
Steam tables introduced in the 1930s served industries for about 30 years, but the need to update the tables was felt in the 1960s when the operating regime of steam power plants began to extend toward higher pressures and temperatures. higher. At that time it became essential to impart more precision and to incorporate accurate experimental data into the table of steam properties. Computers had a tremendous impact in this regard and in 1967 a new formulation, IFC-67, became the next internationally acceptable version.
In the 1990s, the need for further refinement in calculation accuracy, reliability, and speed was strongly felt and in 1995, IAWPS adopted a new version of the properties of water and steam known as IAPWS-95, which was greatly improved. compared to IFC-67. Subsequently, the use of computational speed became an important criterion with the advent of personal computers, and finally, in 1997, the international community formally adopted the latest version known as IAPWS-IF97. IFC-67 was in use for almost 30 years and in the meantime many custom programs were already in use in industry and academia. Therefore, it was not so easy to introduce a completely new formulation in the 1990s.
In IF97, the entire thermodynamic regime has been divided into five regions to facilitate faster calculations and better consistency where the specific formulation for each zone has been detailed. Splitting into multiple zones has certainly produced better consistency and a vehicle for faster computation, but raised questions about output near zone boundaries, which doesn’t really matter. IF97 took special precautions to minimize discontinuities in boundary zones so that calculation results do not significantly affect calculations, especially power cycles.
Advances in IAPWS-IF97
The progress made in IF-97 can be summarized as follows:
Increased calculation speed
Increased accuracy
Improved consistency
Entering the highest temperature zone
It has been observed that, on average, calculations based on IF-97 are about 5 times faster (not in the critical point region) than the formulation used on IF-67. Thanks to the introduction of backward functions, which helped reduce the time required for iterative calculations. However, the actual speed of computation is highly dependent on the skill of the programmer and the power of the computer’s processor, and the effect of speed becomes predominant in computations involving finite element methods. The aspect of precision and consistency has become very important in IF-97 by properly presenting the latest experimental results, etc. and it has been thoroughly tested before its adoption by the international community. The introduction of a new high temperature zone (zone 5) up to 2000 Deg C has helped the design of combined cycle plants. Other advantages of IF-97 are the inclusion of the speed of sound, which has aided in the analysis of throttling flow and density as a function of temperature and pressure, making it easier to calculate viscosity and thermal conductivity.
Effects of adopting IAPWS-IF97
Although not overwhelming, the transition to IF97 from IF67 had significant effects regarding the output obtained using IF97 compared to IF67 and can be summarized below:
Change in latent heat of vaporization
Enthalpy change at superheated temperatures
New formulation for metastable region
The improved accuracy, use of the Gibbs energy equation, and other scientific features of IF97 have provided a more reliable approach to the design and performance of steam-based equipment. The main difference that it has created with respect to the IF67 is in the heat rate calculation, which has commercial importance. Some problems were found for the plants, which were designed based on IF67 and subsequently upgraded after the introduction of IF97 and had to be painstakingly resolved. However, plants designed in the late 1990s and 2000s do not suffer from such controversies as the international community largely accepted IF97.
