Sunday, January 24, 2010

Steam engine

A steam engine is a machine that converts the heat energy of steam into mechanical energy by means of a piston moving in a cylinder. As an external combustion engine—since it burns its fuel outside of the engine—a steam engine passes its steam into a cylinder where it then pushes a piston back and forth. It is with this piston movement that the engine can do mechanical work. The steam engine was the major power source of the Industrial Revolution and dominated industry and transportation for 150 years. It is still useful today in certain situations.

History

The earliest known steam engines were the novelties created by the Greek engineer and mathematician named Hero who lived during the first century A.D. His most famous invention was called the aeliopile. This was a small, hollow sphere to which two bent tubes were attached. The sphere was attached to a boiler that produced steam. As the steam escaped from the sphere’s hollow tubes, the sphere itself would begin to whirl and rotate. Hero of Alexandria and several other Greeks designed many other steam-powered devises, such as a steam organ and automatic doors, but always in the context of playfulness and seemingly without any interest in using steam in a practical way. Nonetheless, their work established the principle of steam power and their playful devices were a real demonstration of converting steam power into some kind of motion.

Although the Greeks established the principle of steam power, it lay ignored for over 1,500 years until the late 1600s in Europe. During this long period, the main sources of power were first, human muscle power or draft animals, and later, wind and water power. Windmills and waterwheels were adequate for slow, repetitive jobs like grinding corn, in which an interruption of power was of little consequence. However, for certain jobs, like pumping water from a mine shaft, a power source that could cease at any time was not always satisfactory. In fact, it was the very deepness of the English mines that spurred engineers to search for pumps that were quicker than the old water pumps. By the mid-sixteenth century, work on air pumps had established the notion of a piston working in a cylinder, and around 1680, the French physicist Denis Papin (1647-1712) put some water at the bottom of a tube, heated it, converted it to steam, and saw that the expanded steam pushed forcibly and moved a piston just ahead of it. When the tube cooled, the piston returned to its previous position. Although Papin was well aware he had created an engine that could eventually do work, he was deterred by the very real mechanical difficulties of his time and chose to work on a smaller scale-creating the world’s first pressure cooker. Following Papin, an English military engineer, Thomas Savery (c.1650-1715), built what most regard as the first practical steam engine. Unlike Papin’s system, this had no piston since Savery wanted only to draw water from the coal mines deep below the earth. Knowing that he could use steam to produce a vacuum in a vessel, he connected such a vessel to a tube leading into the water below. The vacuum then drew water up the tube and blew it out by steam pressure. Savery’s system was called the “Miner’s Friend” as it raised water from the mines using the suction produced by condensing steam. A few years later, an English engineer and partner of Savery named Thomas Newcomen (1663-1729) improved the steam pump by reintroducing the piston. By 1712 he had built an engine that used steam at atmospheric pressure (ordinary boiling water) and which was fairly easy to build. His piston engine was very reliable and came into general use in England around 1725. His machine was called a beam engine because it had a huge rocking-arm or see-saw beam at its top whose motion transferred power from the engine’s single cylinder to the water pump.

Understanding how the Newcomen engine worked provides insight into all later steam engines. First, the entire machine was contained in an engine house, about three stories high, out of whose top wall poked a long oak beam that could rock up and down. The house was constructed off to the side of the mine shaft. At the bottom of the shaft was the water pump which was connected to the engine by a long pump-rod. Below the beam inside the house was a long brass cylinder which sat atop a brick boiler. The boiler was fed by coal and supplied the steam. Inside the cylinder was the piston that could slide up and down and was connected to the beam above. The engine always started with the piston in the up position. Then steam filled the cylinder from an open valve. When filled, the cylinder was sprayed with water which caused the steam inside to condense into water and create a partial vacuum. With this, the pressure of the outside air would force the piston down, which rocked the beam and pulled up the pump rods and sucked up about 12 gal (45 l) of water. The piston then returned to its starting position (up) in the cylinder and the process was repeated. Besides being called a beam engine, Newcomen’s engine was also called an atmospheric engine since it used air pressure to move the piston (down).

The most important improvement in steam engine design was brought about by the Scottish engineer James Watt (1736-1819). In 1763, Watt was asked to repair a Newcomen engine and was struck by what he considered its inefficiency. He set out to improve its performance and by 1769 had arrived at the conclusion that if the steam were condensed separately from the cylinder, the latter could always be kept hot. That year he introduced a steam engine with a separate condenser. Since this kept the heating and cooling processes separate, his machine could work constantly without any long pause at each cycle to reheat the cylinder. Watt continued to improve his engine and made three additions that were highly significant. First, he made it double-acting by allowing steam to enter alternately on either side of the piston. This allowed the engine to work rapidly and deliver power on the downward as well as on the upward piston stroke. Second, he devised his sun-and-planet gearing which was able to translate the reciprocal or to-and-fro motion of the beam into rotary motion. Third, he added a centrifugal governor that maintained a constant engine speed despite varying loads. This highly innovative device marks the early beginnings of automation, since Watt had created a system that was essentially self-regulating. Watt also devised a pressure gauge that he added to his engine. By 1790, Watt’s improved steam engines offered a powerful, reliable power source that could be located almost anywhere.

This meant that factories no longer had to be sited next to water sources, but could be built closer to both their raw materials and transport systems. More than anything, it was Watt’s steam engine that speeded up the Industrial Revolution both in England and the rest of the world. Watt’s steam engine was not perfect however, and did have one major limitation; it used steam at low pressure. High pressure steam meant greater power from smaller engines, but it also meant extreme danger since explosions of poorly-made boilers were common. The first to show any real success with it was the English inventor Richard Trevithick (1771-1833). By the end of the eighteenth century, metallurgical techniques were improving and Trevithick believed he could build a system that would handle steam under high pressure. By 1803, Trevithick had built a powerful, high-pressure engine that he used to power a train. His technical innovations were truly remarkable, but high-pressure engines had earned such a bad reputation in England that twenty years would pass before English inventor George Stephenson (1781-

1848) would prove their worth with his own locomotives.

In the United States however, there was little bias against, or hardly any knowledge of, steam power. Toward the end of the eighteenth century, Evans began work on a high-pressure steam engine that he could use as a stationary engine for industrial purposes and for land and water transport. By 1801, he had built a stationary engine that he used to crush limestone. His major high-pressure innovation placed both the cylinder and the crankshaft at the same end of the beam instead of at opposite ends. This allowed him to use a much lighter beam.

Over the years, Evans built some 50 steam engines which were not only used in factories, but also to power an amphibious digger. High-pressure steam ran this odd-looking scow that was a dredge that could move on land as well as in water. It was the first powered road vehicle to operate in the United States.

Despite Evans’ hard work and real genius, his innovative efforts with steam met with little real success during his lifetime. He was often met with indifference or simple reluctance on the part of manufacturers to change their old ways and convert to steam. His use of steam for land propulsion was set back by poor roads, vested interest in horses, and woefully inadequate materials. After Evans, high-pressure steam became widely used in America, unlike England where Watt’s low-pressure engines took a long time to be replaced. But improvements were made nonetheless, and iron would eventually replace timber in engine construction, and horizontal engines came to be even more efficient than the old vertical ones.

The workings of a steam engine

Throughout all of this development and improvement of the steam engine, no one really knew the science behind it. Basically, all of this work had been accomplished on an empirical basis without reference to any theory. It was not until 1824 that this situation changed with the publication of Reflexions sur La Puissance Motrice du Feu by the French physicist, Nicolas Leonard Sadi Carnot (1796-1832). In his book On the Motive Power of Fire, Carnot founded the science of thermodynamics (or heat movement) and was the first to consider quantitatively the manner in which heat and work are related. Defining work as “weight lifted through a height,” he attempted to determine how efficient or how much “work” a Watt engine could produce. Carnot was able to prove that there was a maximum theoretical limit to the efficiency of any engine, and that this depended upon the temperature difference in the engine. He showed that for high efficiency, steam must pass through a wide temperature range as it expands within the engine. Highest efficiency is achieved by using a low condenser temperature and a high boiler pressure. Steam was successfully adapted to power boats in 1802 and railways in 1829. Later, some of the first automobiles were powered by steam, and in the 1880s, the English engineer Charles A. Parsons (1854-1931) produced the first steam turbine. This high-powered, highly efficient turbine could produce not only mechanical energy but electrical energy as well. By 1900, the steam engine had evolved into a highly sophisticated and powerful engine that propelled huge ships in the oceans and ran turbogenerators that supplied electricity.

Once the dominant power source, steam engines eventually declined in popularity as other power sources became available. Although there were more than 60,000 steam cars made in the U. S. between 1897 and 1927, the steam engine eventually gave way to the internal combustion engine for vehicle propulsion. Today, interest in steam has revived somewhat as improvements make it increasingly efficient and its low-pollution factors make it more attractive.

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