Unit 3 modern high-pressure steam engines
LEAD-IN
Where are steam engines used today? Why is their use restricted mainly by atomic power plants and thermo-electric power stations?
2. In a modern steam engine, the slide valve alternately routes steam to the back and front of the cylinder to drive the piston. Study the diagram which shows some of the important components of a steam engine. Then, use the diagram and the words and phrases below to describe a steam engine complete cycle of operation depicted in Figures 1a through 1d.
Figure 1a |
piston, at the left end of the cylinder, valve chest, to flow through the port, cylinder, slide valve, used steam, to escape through the exhaust port, to drive a flywheel, to drive the rod, to control the slide valve |
Figure 1b |
steam, at the left side of the cylinder, to expand, to move the piston, the valve, to move to the closed position, to seal the cylinder, to escape |
Figure 1c |
to move toward the right, the pressure of the expanding steam, the port at the left end of the cylinder, to be connected to the exhaust, the valve chest, to contain steam, to be connected to the right end of the cylinder |
Figure 1d |
valve, to cover the ports, from both ends of the cylinder, piston, to move toward the left, to be driven by the expansion of steam, at the right end of the cylinder |
READING
3. Read the text and match the sentences (a-c) to the numbered spaces (1-3) in the text.
A. Owing to technological advances and the use of high-temperature steam, steam turbines have attained an efficiency of thermal energy conversion of approximately 40 percent.
B. 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.
C. He built a stationary high-pressure steam engine for driving a rotary crusher to produce pulverized limestone for agricultural use.
Modern high-pressure steam engines
Although Watt understood the advantages of utilizing the expansive power of steam within a cylinder, he refused to use steam under high pressure for reasons of safety. The pressures in his engines were not much greater than normal atmospheric pressure, or about 15 pounds per square inch (p. s. i.), or 103 kilopascals. This limited the application of steam engines. By 1815, Oliver Evans, an American, had built an engine that used 200 p. s. i. (1,379 kilopascals) of pressure. _ 1 _ Within a few years Evans designed lighter-weight high-pressure steam engines that could do various other tasks, such as drive sawmills, sow grain, and power a dredge. From 1806 to about 1816 he produced more than 100 steam engines that were employed with screw presses for processing paper, cotton, and tobacco. Today, many engines use steam under a pressure of more than 1,000 p. s. i. (6,895 kilopascals).
Other major advances in the use of high-pressure steam were achieved by Richard Trevithick in England during the early years of the 19th century. One of his first engines operated under 30 p. s. i. (207 kilopascals) of pressure. Trevithick built the world's first steam-powered railway locomotive in 1803. Two years later he adapted his high-pressure steam engine to drive an iron-rolling mill and to propel a barge with the help of paddle wheels.
Watt’s engine was able to convert only a little more than 2 percent of the thermal energy in steam to work. The improvements introduced by Evans, Trevithick, and others (e.g., three separate expansion cycles and higher steam temperatures) increased the efficiency of the steam engine to roughly 17 percent by 1900. Yet, within the next decade the steam engine was supplanted for various important applications by the more efficient steam turbine. _ 2 _
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.
_ 3 _ In this engine, steam moves in only one direction while entering the cylinder of the engine, expanding, and then leaving the cylinder. 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.
From Encyclopædia Britannica
LANGUAGE DEVELOPMENT