Thermodynamic Properties of Water and Steam for Power Generation

In the early 1900s, the electric power generation industry was experiencing rapid growth and change. The steam engines used for power in the previous century had been displaced by turbines which generated electricity as they were rotated by pressurized steam generated in boilers. Turbines and boilers were operating at higher temperatures and pressures (and also in increasingly complex cycles, which required more sophisticated thermal design and analysis) in order to attain greater thermodynamic efficiency. They were also becoming larger as the demand for electricity skyrocketed. A major source of growing pains for the industry was the lack of accurate and standardized values for the properties of water and steam. For the design of power plants and the boilers and turbines within them, it is necessary to have accurate values of thermodynamic quantities such as the vapor pressure (pressure at which water boils at a given temperature) and the enthalpy of vaporization or latent heat (amount of heat required to generate steam from liquid water). More important, the evaluation of the performance of purchased equipment depends on the calculation of these properties. The efficiency of a turbine is measured as the fraction of the energy available in the steam that is converted to electricity, but that available energy is calculated to be a different number depending on the values used for the thermodynamic properties. A turbine might appear to be 28 % efficient with one set of properties and only 27 % efficient with another set; because of the large flows involved, these small differences could mean large sums of money. It therefore became imperative to settle on internationally standardized values for the properties of water and steam, so that all parties in the industry could have a “level playing field” on which to compare bids and equipment performance. The paper Calorimetric Determination of the Thermodynamic Properties of Saturated Water in both the Liquid and Gaseous States from 100 to 374 C [1] describes the accurate measurements carried out at NBS that were essential in reaching agreement on the needed standards. In the United States, this problem was first addressed in 1921 by a group of scientists and engineers brought together by the American Society of Mechanical Engineers (ASME). The 1921 meeting led to the formation of the ASME Research Committee on Thermal Properties of Steam. This committee, recognizing the need for reliable data, collected subscriptions from industry and disbursed the money to support experimental measurement of key properties of water and steam at Harvard, MIT, and the Bureau of Standards. Because the ASME committee was not as successful in their fundraising as they had hoped, all three institutions ended up subsidizing some of the research themselves in recognition of its importance. The need for standard, reliable data was also recognized in other countries (notably England, Germany, and Czechoslovakia), and research efforts were coordinated internationally. In the late 1920s and early 1930s, three international conferences were held with the purpose of agreeing on standardized values for the properties of water and steam. This culminated in 1934 with the adoption of a standard set of tables, covering the range of temperatures and pressures of interest to the power industry at that time. These tables gave the vapor pressure as a function of temperature, values of the volume and enthalpy for the equilibrium vapor and liquid phases along the vapor-pressure curve, and volumes and enthalpies at points on a coarse grid of temperatures and pressures. Each value had an uncertainty estimate assigned to it. The data those tables were based on also became the basis for a book of “Steam Tables” produced by J. H. Keenan and F. G. Keyes [2]; the Keenan and Keyes tables were the de facto standard for the design and evaluation of steam power generation equipment worldwide for the next 30 years. The most important data behind these new steam tables came from the laboratory of Nathan S. Osborne at the National Bureau of Standards. Through most of the 1920s and 1930s, Osborne and his coworkers painstakingly built equipment and conducted measurements. The ASME had originally hoped for data within three years of the project’s 1921 start, but fortunately they were patient (and grateful to the NBS for subsidizing the work) and continued to support the project through years of pioneering, but often frustrating, apparatus development. Finally, beginning in the late 1920s, their patience was rewarded as data of unparalleled quality began coming from Osborne’s laboratory. The primary experimental technique was calorimetry, in which a measured amount of heat is added to a fluid under controlled conditions. Osborne and coworkers had previously performed calorimetric measurements on ammonia. For measurements on water, several new