Background About IAPWS-IF97, the New Industrial Formulations for Steam Properties
Since 1967, the industry standard for the thermodynamic properties of steam and water has been the IAPWS "1967 IFC-Formulations for Industrial Use". These formulations have been the basis for the American Society of Mechanical Engineers (ASME) steam tables and also for Techware’s WinSteam product. In 1995, The International Association for the Properties of Water and Steam (IAPWS) released a new formulation for scientific use called the "IAPWS Formulation 1995 for the Thermodynamic Properties of Ordinary Water Substance for General and Scientific Use", otherwise known as the IAPWS-95 formulation. While these formulations are the most accurate representation of the thermodynamic properties of steam and water available to date, they are considered to be too complex and time consuming for industrial use. As a result, IAPWS released a new industrial formulation in 1997 called IAPWS-IF97, which is derived from IAPWS-95. In January 1999, the ASME adopted IAPWS-IF97 as the ASME Steam Properties for Industrial Use.
It is interesting to note that while IF-97 offers some improvements in accuracy over IFC-67, the driving force behind the development of the new formulation was increased computing speed. This effort was promoted by the power industry, which makes extensive use of steam properties in preparing power cycle calculations. The IFC-67 is based on a complex structure of equations and requires numerous iterations for many property calculations. In all regions except the critical region, the thermodynamic properties are calculated as a function of pressure and temperature. Approximately 60% of the steam property calculations in power cycles are determined by other input combinations and therefore require numerous iterative calculations under IFC-67. One of the speed improvements in IF-97 stems from the introduction of backward functions, which calculate temperature as a function of pressure and enthalpy or entropy.
Other than speed improvements, IF-97 offers the following changes:
- Improved accuracy – since IF-97 is derived from IAPWS-95, it provides closer correspondence with the accurate scientific formulations than does IFC-67.
- New Temperature Scale – IFC-97 is based on the International Temperature Scale of 1990 (ITS-90), while IFC-67 is based on the International Practical Temperature Scale of 1968 (IPTS-68). Between 0 deg C and 800 deg C, the maximum difference between ITS-90 and IPTS-68 is 0.36 deg C, although the average difference is much smaller.
- New High Temperature Region – IF-97 provides thermodynamic properties in region 5, a new region extending from temperatures between 800 deg C to 2000 deg C and pressures up to 50 MPa. (Revised in 2007)
- Improved Consistency at Boundaries – IF-97 provides better property continuity at the boundaries between the regions than IFC-67.
Techware has released WinSteam 3.0, an upgrade to WinSteam based on IF-97 in order to be consistent with the new standard. WinSteam 4.0 includes the extension of Region 5 to 50 MPa. Whereas the major benefit of IF-97 is improved computational speed, this is of minor importance to most WinSteam users who need thermodynamic steam properties on their spreadsheet. Because of the new temperature scale and the improved accuracy of IF-97, there will be differences in values calculated using the new version. While these differences are small in most instances, they may cause inconvenience for customers using the new formulations with existing spreadsheets. To avoid this difficulty, WinSteam 3.0 defaults to the IF-97 formulations but allows the user to select the IFC-67 formulations by adding a modifier to the unitset parameter. Function names have not changed to allow easy conversion to the new formulations.