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Steam and water properties international formulation

Properties of steam and water take key roles in the design and performance of steam turbines, boilers, steam heaters and other steam driven equipment. Small error in the calculation of major properties, like enthalpy, can generate huge loss for the performance of boilers, turbines etc. These issues attach major weight to the correctness of the steam tables, and steam and water properties software.

During 1920s and 1930s, International engineering community in 1920s badly felt the requirement of a commonly accepted table for the steam and water properties, which will be binding upon the purchasers and suppliers. After a series of deliberations they agreed on some tables of properties and published them as ‘steam tables’ in 1934. Those who were engaged in prolonged discussion and in formulation of the first published steam tables, formed the organization IAWPS or the International Association for the Properties of Water and Steam.

<Such steam tables of 1934 worked successfully for the steam power industry for 30 years. But during 1960s, the necessity for up gradation of the steam tables of 1934 became evident when steam power plants began to operate at higher pressure and temperatures demanding more accuracy and speed of computation. Moreover with the introduction of electronic computers IAWPS was forced to release a new formulation in 1967 known as IFC-67 to the international community.

The demand for more accuracy, reliability and increase in speed of computation endeavored IAWPS in 1995 to introduce another new version known as IAWPS-95, which was greatly improved in comparison to IFC-67. However, rapid development in computing industry and the further demand for more speedy computation led IAWPS to release the latest version of the steam and water properties formulation called IAPWS-IF97 in 1997. It was not easy for the industry to accommodate a new version after long 30 years when software based on IFC-67 was already in commercial use.

The focal point of IF97 was to divide the thermodynamic plane divided into five regions to assist speedy computations and improved consistency where separate formulations for each zone were provided. The creation of five zones resulted in better consistency and quick calculation but forced authorities to take special care to reduce the discontinuities in boundary zones so that protect the accuracy of the power cycle calculations. 

Improvements in IAPWS-IF97

Improvements achieved in adopting IF-97 are as under:

1. Gain in speed of computation  2.Better accuracy 3. Improved Consistency 4. Increase in the range of temperature up to 2000 Deg C

Calculations using IF-97 are about 5 times faster (not in critical point region) in comparison to the formulation of IF-67 owing to the introduction of backward functions for reducing the time for iterative calculations. The computation speed also depends upon the programmer’s ability and computer processor’s power. The tangible effect of computational speed becomes more evident in computations requiring finite element methods. Accuracy and consistency aspect has contributed in a great way in IF-97 by incorporating more research input after has been thorough testing prior to acceptance by international authorities. New high temperature zone 5 that covers up to 2000 Deg C, has given a boost in the design of combined cycle power stations. Other advantages of IF-97 are Inclusion of speed of sound has facilitated the choke flow analysis and introduction of density as a function of temperature and pressure helped in the computation of viscosity and thermal conductivity.

Effects of adopting IAPWS-IF97

The transition from IF67 to IF97 had considerable effects with respect to output obtained as listed herein under:

1. Revisions in latent heat of vaporization 2. Revisions in enthalpy at superheated temperatures 3. Introduction of new formulation for metastable region

Enhanced accuracy, utilization of Gibbs energy equation and other research inputs have given IF97 a more trustworthy advancement to the design and performance of steam driven machinery. The major difference it has prompted with respect to IF67 is in computation of heat rate, which has tremendous commercial value. During the transition period, few problems were faced for the plants, which were basically designed using IF67 and later modernized after the release of IF97 and those were to be resolved with pain. Subsequent plants designed after 1997 generally do not suffer from major controversies as the international engineering and scientific community broadly accepted the IF97 formulation.



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