Steam and Water Properties Formulation History

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Steam and water properties play pivotal roles in the design and performance of steam boilers, steam turbines, and other steam related equipment. Even little discrepancies in the determination of key properties, such as enthalpy, can lead to loss of thousands of dollars for the performance of turbines or boilers. These aspects impart tremendous importance to the accuracy of steam tables, which is mostly replaced nowadays by the computerized steam and water properties software.

During 1920s and 1930s, engineers and scientists desperately felt the need to agree on an internationally acceptable table, which should be reasonably accurate, for the steam and water properties that can be lawfully binding on the purchasers and suppliers. After many debates and long discussions international scientific and engineering community agreed on few 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 birth to the organization called the International Association for the Properties of Water and Steam, or IAPWS.

The steam tables introduced in 1930s served the industries for about 30 years, but the need to upgrade the tables felt in 1960s when steam power stations operating regime started to be extended towards higher pressure and temperatures. By that time it became essential to impart more accuracy and to incorporate accurate experimental data into the steam properties table. Computers made tremendous impact in this regard and in 1967 a new formulation, IFC-67, became next version to be internationally acceptable.

In 1990s the need for further refinement in accuracy, reliability and speed of computation was highly felt and in 1995, IAWPS adopted a new version of steam and water properties known as IAPWS-95, which was vastly improved in comparison with IFC-67. Subsequently, the use of computational speed became a major criterion with the advent of personal computers and lastly in 1997, the latest version known as IAPWS-IF97 was formally adopted by the international community. IFC-67 was in use for nearly 30 years and meanwhile many tailor-made software were already in use in industry and academic world. Hence it was not so simple to introduce an altogether new formulation in 1990s.

In IF97, entire thermodynamic regime has been divided into five regions to facilitate faster computations and better consistency where specific formulation for each zone has been spelt out. The division into multiple zones, no doubt, has produced better consistency and a vehicle for faster calculation but raised questions about the output near zone boundaries, which in reality has no significance. IF97 took special precautions to minimize the discontinuities in boundary zones so that the computation results do not significantly affect calculations, especially the power cycles.

Advancements in IAPWS-IF97

Advancements achieved in IF-97 can be summarized as below:

· Increase in speed of computation

· In crease in accuracy

· Improved Consistency

· Introduction of higher temperature zone

It has been observed that on an average, calculations based on IF-97 are about 5 times faster (not in critical point region) than the formulation used in IF-67. Thanks to the introduction of backward functions, which helped in reducing the time taken in iterative calculations. However, the real life speed of computation vastly depends upon the programmer’s skill and computer processor’s power and the effect of speed becomes pre-dominant in computations involving finite element methods. Accuracy and consistency aspect has assumed major significance in IF-97 by aptly introducing latest experimental results etc. and the same has been thoroughly tested before adoption by international community. Introduction of 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 inclusion of speed of sound, which has helped in choke flow analysis and density as a function of temperature and pressure, which facilitates the computation of viscosity and thermal conductivity.

Effects of adopting IAPWS-IF97

Although not overwhelming, the transition to IF97 from IF67 had significant effects as with respect to output obtained using IF97 compared to IF67 and those can be summarized below:

· Change in latent heat of vaporization

· Change in enthalpy at superheated temperatures

· New formulation for metastable region

The improved accuracy, use of Gibbs energy equation and other scientific features of IF97 have given more reliable approach to the design and performance of steam based equipment. The main difference it has created with respect to IF67 is in calculation of heat rate, which has commercial significance. Some problem was encountered for the plants, which were designed based on IF67 and subsequently upgraded after the introduction of IF97 and those had to be solved with pain. However, the plants designed in late 1990s and in 2000s do not suffer from such controversies since international community largely accepted IF97.

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AUTOPOST by BEDEWY VISIT GAHZLY