Wednesday, January 11, 2012

Steam Engine


Steam Engine

Steam Engine, mechanical device used to transfer the energy of steam into mechanical energy for a variety of applications, including propulsion and generating electricity. The basic principle of the steam engine involves transforming the heat energy of steam into mechanical energy by permitting the steam to expand and cool in a cylinder equipped with a movable piston. Steam that is to be used for power or heating purposes is usually generated in a boiler. The simplest form of boiler is a closed vessel containing water, which is heated by a flame so that the water turns to saturated steam. The ordinary household-heating system usually has a boiler of this type, but steam-generating plants used for power purposes are more complex in design and are equipped with various auxiliary devices. The efficiency of a steam engine is generally low, and therefore, in most power generation applications, the steam engines have been replaced by steam turbines (see Turbine).
II
HISTORY
Parts of a Steam Engine
Harnessing the power of steam marked a significant step in technology. The introduction of the steam engine led to many new inventions, most notably in transportation and industry. Steam engines transfer the energy of heat into mechanical energy, often by allowing steam to expand in a cylinder equipped with a movable piston. As the piston moves up and down (or alternatively, from side to side), an attached arm converts this motion into parallel motion that drives a wheel. Models of the steam engine were designed as early as 1690, but it was not until 70 years later that James Watt arrived at the design of the modern steam engine.

The first piston engine was developed in 1690 by the French physicist and inventor Denis Papin and was used for pumping water. Papin's engine, which was little more than a curiosity, was a crude machine in which the actual work was done by air rather than steam pressure. It consisted of a single cylinder that also served as a boiler. A small amount of water was placed in the bottom of the cylinder and heated until steam was formed. The pressure of this steam raised a piston fitting in the cylinder, and, after it was raised, the source of heat was removed from the bottom of the cylinder. As the cylinder cooled, the steam condensed and air pressure on the upper side of the piston forced the piston down.
In 1698, the English engineer Thomas Savery built a steam engine that used two copper vessels alternately filled with steam from a boiler. Savery's engine was used for pumping water, but could only raise water about 6 m (20 ft) without using pressures which risked explosion, and was quickly abandoned. The first practical steam engine, the so-called atmospheric engine, was built by the English inventor Thomas Newcomen in 1712. This device had a vertical cylinder and a piston that was counterweighted. Steam admitted to the bottom of the cylinder at very low pressure acted with the counterweight to move the piston to the top of the cylinder. When the piston reached this point, a valve opened automatically and sprayed a jet of cold water into the cylinder. The water condensed the steam, and atmospheric pressure forced the piston back to the bottom of the cylinder. A rod attached to the arm of the pivoted beam that connected piston and counterweight moved up and down as the piston moved, actuating a pump. Newcomen's engine was not efficient, but it was sufficiently practical to be used extensively for pumping water from coal mines.
In the course of making improvements to the Newcomen engine, the Scottish engineer and inventor James Watt produced a series of inventions that made possible the modern steam engine. Watt's first important development was the design of an engine that incorporated a separate condensing chamber for the steam. This engine, patented in 1769, greatly increased the economy of the Newcomen machine by avoiding the loss of steam that occurred in alternate heating and cooling of the engine cylinder. In Watt's engine, the cylinder was insulated and remained at steam temperature. The separate condenser chamber, which was water-cooled, was equipped with a pump to maintain a vacuum so that the steam was drawn from the cylinder to the condenser. The pump was also used to remove the water from the condenser chamber.
Another radical departure in the design of the early Watt engines was the use of steam pressure instead of atmospheric pressure to perform the actual work of the engine. Watt also devised a method in which the reciprocating pistons of engines drove a revolving flywheel. He accomplished this first by a system of gearing (see Gear), and later by means of a crankshaft, as in modern engines. Watt's other improvements and inventions included application of the principle of double action, whereby steam was admitted to each end of the cylinder alternately to drive the piston back and forth. He also equipped his engines with throttle valves to control speed and also with governors in order to maintain automatically a constant speed of operation.
The next important development in the field of steam engines was the introduction of practical noncondensing engines. Although Watt had recognized the principle of the noncondensing engine, he had been unable to perfect machines of this type, probably because he used steam at extremely low pressure. At the beginning of the 19th century the British engineer and inventor Richard Trevithick and the American inventor Oliver Evans devised successful noncondensing engines using the high-pressure steam. Trevithick used this model of steam engine to power the first railroad locomotive ever made (see Locomotive). Both Trevithick and Evans also built steam-powered carriages for road travel.
At about the same time, the first compound steam engines were built by the British engineer and inventor Arthur Woolf. In the compound engine, steam at high pressure is used in one cylinder and then, after it has expanded and consequently lessened in pressure, is piped to another cylinder, in which it expands still further. Woolf's original engines were of the two-cylinder type, but later types of compound engines used triple and even quadruple expansion. The advantage of compounding two or more cylinders is that less energy is lost in the heating of the cylinder walls; as a result, the engine is more efficient.
III
MODERN STEAM ENGINES
Steam Engine: Figures 1a-d
In a steam engine, the slide valve alternately routes steam to the back and front of the cylinder to drive the piston. The diagram at right shows some of the important components of a steam engine, while Figures 1a through 1d depict a steam engine’s complete cycle of operation.

The operation of a typical modern steam engine is depicted in Figures 1a-d, which show a steam engine's cycle of operation. In Fig. 1a, when the piston is at the left end of the cylinder, steam admitted to the valve chest flows through the port into the cylinder at the left-hand side of the piston. The position of the slide valve also allows the used steam at the right-hand end of the cylinder to escape through the exhaust port. The piston's motion drives a flywheel, which in turn drives the rod that controls the slide valve. The relative positions of the piston and the slide valve are governed by the relative positions of where the crankshaft and the slide valve rod are fastened to the flywheel.
In the second position, shown in Fig. 1b, the steam at the left side of the cylinder has expanded and moved the piston to the center point of the cylinder. At the same time, the valve has moved to the closed position so that the cylinder is entirely sealed and neither the steam in the cylinder nor the steam in the valve chest can escape.
As the piston continues to move toward the right under the pressure of the expanding steam, as shown in Fig. 1c, the port at the left end of the cylinder is connected to the exhaust by the valve, and, at the same time, the valve chest, which contains steam, is connected to the right end of the cylinder. In this position, the engine is prepared for the second stroke of its double-action cycle. Finally, in the fourth position (Fig. 1d), the valve again covers the ports from both ends of the cylinder, and the piston moves toward the left, driven by the expansion of steam at the right end of the cylinder.
The type of valve illustrated in the figure is the simple slide valve, which is the basis for most valves used on modern steam engines. Such valves have the advantage of being reversible; their position relative to the piston can be varied by varying the position of the eccentric that drives them, as shown in Fig. 2. Moving the eccentric through 180° makes it possible to reverse the direction of rotation of the engine.
The slide valve, however, has a number of drawbacks, the most important of these being the friction caused by steam pressure on the back of the valve. To avoid the wear caused by this pressure, steam-engine valves are frequently made in a cylindrical form entirely enclosing the piston, so that the pressure is equal all around the valve and friction is minimized. The development of this type of valve is attributed to the American inventor and manufacturer George Henry Corliss. In other types of slide valves, the moving portion of the valve is designed so that steam does not press directly on the back of the valve.
The linkage between the piston of the engine and the valve supplying steam to the engine is very important in determining the power and efficiency of an engine. By varying the point in the engine cycle at which steam is admitted to the cylinder, it is possible to vary the amount of compression and expansion in the cylinder and hence to vary the power output of the engine. A number of different types of valve gears for linking the piston to the valve have been developed that permit not only reversing of the engine, but also a range of control of the admission time and cutoff of the steam. Valve gears are of particular importance in steam locomotives in which the effort required from the engine varies widely; the effort is at a maximum when the locomotive is starting and less when it is running at full speed.
An important adjunct to all types of reciprocating steam engines is the flywheel, which is driven by the piston crank. Because of its inertia, the flywheel, usually a heavy metal casting, makes continuous the individual surges of power of the steam expanding within the cylinder, and permits the engine to provide a uniform flow of power.
In single-cylinder steam engines, the engine can stop when the piston is at one end of the cylinder or the other. If the cylinder is in this position, the engine is said to be on dead center and is impossible to start. To eliminate the dead-center points, steam engines are frequently equipped with two or more coupled cylinders, arranged in such a way that no matter what the position of the pistons, the engine is able to start. The simplest way of coupling two cylinders in an engine is to arrange the two cranks on the flywheel as shown in Fig. 3. For better balance, it is also possible to use a three-cylinder engine in which the various cranks are set at an angle of 120°. The coupling of engines not only eliminates difficulties in starting but also produces a power plant that operates more reliably.
The cylinder of a compound engine, unlike that of a single-cylinder engine of the ordinary type, can be kept close to a uniform temperature, which makes the engine more efficient.
Further improvement in the design of steam engines is afforded by the uniflow engine, which uses the piston itself as a valve and in which all portions of the cylinder remain at approximately the same temperature when the engine is operating. In the uniflow engine, steam moves in only one direction while entering the cylinder of the engine, expanding, and then leaving the cylinder. This unidirectional flow is accomplished by employing two sets of inlet ports at either end of the cylinder, together with a single set of outlet ports in the cylinder wall at the center. The flow of steam into the two sets of inlet ports is controlled by separate valves. The inherent advantages of the uniflow system are such that engines of this type were usually chosen for use in large installations, although the initial cost of the engines is considerably higher than that of conventional steam engines. One virtue of the uniflow engine is that it permits the efficient use of high-pressure steam in a single cylinder engine without the necessity of compounding.

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