chapter  7
12 Pages

Electricity from steam

By the late nineteenth century, the steam engine had been brought to a high state of development, providing another approach to reliable operation of electricity generators. But, in this application, steam engines have several disadvantages: The reciprocating motion of the pistons needs to be converted to the rotary motion of the generator. This is inefficient and produces significant vibration. Reciprocating steam engines are slow. As the nineteenth century drew to a close, a major need of the rising electrical industry was an engine that could both run at very high speeds and provide smooth, vibration-free operation. A turbine in which steam is the working fluid-the steam turbine-meets both requirements, and soon took over the job of driving electricity generators. The English engineer Charles Parsons was a pioneer in the

development of turbines that used steam as the working fluid. Parsons’ steam turbine used a sequence of fixed guide blades and moving blades. Steam flows between the guide vanes, so that it meets the blades of the wheel at right angles. Steam was admitted at the middle, and flowed parallel to the axle in both directions, delivering kinetic energy to the turbine as it expanded to atmospheric pressure. The electricity generator was directly connected to the turbine axle. Flow of steam through the turbine was assisted by

a condenser that drew off the exhaust steam. In the condenser the steam, after doing work on the turbine, i.e., keeping it spinning, was condensed at atmospheric pressure. The condensed steam (liquid water) could be disposed of or pumped back to the boiler. Development of this machine for commercial application in the electricity industry took place quickly, so that steam turbines came into widespread use in electricity stations by the end of the nineteenth century. The general style of the Parsons turbine has not been significantly changed since the 1880s, though there are now many variants of this type. One change has been a remarkable improvement in efficiency. The first Parsons unit, generating 7.5 kW, has evolved into gigantic machines that develop 450 MW at a fraction of the fuel consumption of earlier steam engines. In the sixteenth and seventeenth centuries scientists began to

understand the properties of gases. Charles and Gay-Lussac showed that, at constant pressure, the volume of a given quantity of gas is directly proportional to its temperature. In the early 1660s, two English scientists, Henry Power and Richard Towneley, observed that the pressure and volume of a gas are inversely related, provided the temperature is held constant. This relationship can be expressed in mathematical terms as

Pressure Volume = Constant

Their work was known to several of the eminent scientists of the time, including Robert Boyle, Robert Hooke, and Isaac Newton. In France, Edmé Mariotte, presumably unaware of Boyle’s publications, published this same relationship without giving credit to Boyle, let alone to Power and Towneley. By a quirk of history, this law of gas behavior is known by Boyle’s name in English-speaking countries, and by Mariotte’s name in France, but it really should be known as Power and Towneley’s law. The explanation of Boyle’s law derives from the molecular

nature of gases. Pressure exerted by a gas confined in a container is due to gas particles bouncing off the walls of the container. Each particle subjects the wall to a tiny force as it collides and bounces.