Steam Engine Essay, Research Paper
The steam engine is a machine used for converting heat energy into mechanical energy,
using steam as the conversion medium. When water is boiled into steam its volume increases
about 1,600 times, producing a force that can be used to move a piston back and forth in a
cylinder. The piston is attached to a crankshaft that converts the piston’s back-and-forth motion
into rotary motion for driving machinery. From the Greek inventor hero of Alexandria to the
Englishman Thomas Newcomen, many persons contributed to the work of harnessing steam.
However, James Watt’s steam engine, patented 1769 offered the first practical solution by
providing a separate chamber for condensing the steam and by using steam pressure to move the
piston in both directions. These and other improvements by Watt prepared the steam engine for a
major role in manufacturing and transportation during the Industrial Revolution. Today steam
engines have been largely replaced by more efficient devices, for example, the steam turbine, the
electric motor, the international combustion.
Since the early 1900s, steam turbines have replaced most steam engines in large
electric-power plants (see ). Turbines are more efficient and more powerful than steam engines.
In most areas, steam locomotives have been supplanted by more reliable and economical
diesel-electric locomotives. Early steam automobiles have been superseded by cars powered by
lightweight, convenient, and more powerful gasoline and diesel engines. Because of all this,
steam engines today generally are regarded as museum pieces. Nonetheless, the invention of the
steam engine played a major role in the Industrial Revolution by creating a society less
dependent on animal power, waterwheels, and windmills
In 1690 the first steam piston engine was developed by French physicist Denis Papin for
pumping water. In this crude device a small amount of water was placed in a single cylinder over
a fire. As the water evaporated, the steam pressure forced a piston upward. The heat source was
then removed, allowing the steam to cool and condense. This created a partial vacuum (a
pressure below that of the atmosphere). Because the air located above the piston was at a higher
pressure (at atmospheric pressure), it would force the piston downward, performing work. More
practical devices powered by steam were the steam pump–patented in 1698 by the English
engineer Thomas Savery–and the so-called atmospheric steam engine–first built in 1712 by
Thomas Newcomen and John Calley. In the Newcomen engine, steam generated in a boiler was
fed into a cylinder located directly above the boiler. A piston was pulled to the top of the
cylinder by a counterweight. After the cylinder was filled with steam, water was injected into it,
causing the steam to condense. This reduced the pressure inside the cylinder and allowed the
outside air to push the piston back down. A chain-beam lever linkage was connected to a pump
rod, which lifted the pump plunger as the piston moved downward. Some modified Newcomen
engines were in service as late as 1800.
The Scottish instrument maker James Watt noticed that use of the same chamber for
alternating hot steam and cold condensate resulted in poor fuel utilization. In 1765 he devised a
separate water-cooled condenser chamber. It was equipped with a pump to maintain a partial
vacuum and periodically steam was fed from the cylinder through a valve. Watt and his business
partner, Matthew Boulton, sold these engines on the basis that one third of the fuel savings be
paid to them. The fuel costs for the Watt and Boulton engines were 75 percent less than those for
a similar Newcomen engine. Among Watt’s many other improvements was the crankshaft, which
was used to produce rotating power; the use of double-acting pistons, by which steam was fed
alternately into the top and bottom sections of the piston-cylinder assembly to nearly double the
power output of a given engine; a governor, which regulated the flow of steam to the engine; and
the flywheel, which smoothed out the jerky action of the cylinders. Watt also recognized that
using high-pressure steam in the engine would be more economical than using steam at external
atmospheric pressure. Due to limitations in boiler design, however, his engines never operated at
high pressures.
Engines were further improved after the development of boilers that could operate at
higher pressures. By the end of the 18th century, two types of high-pressure boilers were in use:
water-tube boilers and fire-tube boilers. Their shells were made of iron plates fastened together
with rivets. In water-tube boilers, water was heated in coiled or vertical tubes that ran through
the fire chamber and received heat from the hot combustion gases. The steam would collect at
the top of the boilers. These boilers were the precursors of modern power-plant boilers. In
fire-tube boilers, the water was maintained in the lower portion of a large shell. The shell was
traversed by large pipes through which the combustion products passed from the fire grates to
the stack. Again, the steam collected at the top.
With improved boiler design, the British engineer Richard Trevithick built a
noncondensing steam-driven carriage in 1801 and the first steam locomotive in 1803, though its
boiler later exploded. In 1829 George Stephenson built his successful Rocket locomotive. It
contributed to the rapid expansion of railroads in Great Britain and, later, in other countries.
Steam propulsion of ships was tried successfully in 1787 by the American John Fitch,
who placed a steamboat on the Delaware River. In 1807 the American Robert Fulton built a
side-wheel paddle steamer called the Clermont. Equipped with a Watt and Boulton engine,
Fulton’s Clermont, which was more economically successful than Fitch’s endeavors, traveled
from New York City to Albany, ushering in the age of steamships.
At about the same time, noncondensing engines were also being developed by the
American inventor Oliver Evans. Largely due to Evans’ initiative, high-pressure steam was
adopted in the United States much more readily than in Europe, though sometimes with
disastrous results. A large number of boiler explosions plagued river shipping in the United
States throughout much of the early 1900s.
The British inventor Arthur Woolf recognized that more power could be obtained from a
stationary engine by compounding–that is, by expanding the steam only partially in the first
cylinder and then further, to below atmospheric pressure in a second cylinder before passing it to
the condenser. As steam pressures continued to increase, such compound engines eventually
changed from double- to triple- and quadruple-compounding. The most famous engine of the
19th century was the twin-cylinder Corliss engine presented by George Corliss at the 1876
Centennial Exhibition in Philadelphia. Its cylinders were 40 inches (102 centimeters) in
diameter. Its stroke, the maximum distance of piston travel, was 10 feet (3 meters) and its
flywheel was 30 feet (9 meters) in diameter. Turning at 36 revolutions per minute, the Corliss
engine delivered 1,400 horsepower (1,044 kilowatts) to drive the 8,000 machines in Machinery
Hall. Within a decade a marine engine delivering more than 10,000 horsepower (7,460
kilowatts) had been built. Steam-engine development continued actively for another 50 years.
In 1897 the first automobiles to be driven successfully by noncondensing, steam-driven
engines were built by Francis E. and Freelan O. Stanley in Newton, Mass. (see ). These
steam-driven cars were more powerful than the first gasoline-driven vehicles. They eventually
used boiler pressures of up to 1,000 pounds per square inch (6,895 kilopascals). Although
condensers had been added by 1915, steam-driven automobiles were to face their demise shortly
thereafter, largely due to the engine’s enormous weight, low efficiency, and constant need of
attention.
Before the advent of small electric motors, steam engines powered most manufacturing plants. A
single, centrally located engine delivered power to machines by means of shafts, pulleys, and
belts. Farms in the United States used steam-powered tractors. Self-propelled steam-driven
threshing machines moved from farm to farm during the harvesting season until they were
replaced by gasoline- or diesel-driven units.
Steam engines eventually became too large, heavy, and slow to meet the steadily increasing
demand for more power from a single unit. Following the successful design of the more
powerful and compact steam turbine by the British engineer Charles A. Parsons in 1884 and its
application to marine propulsion in 1897, the fate of the large steamship engines was sealed,
though such engines continued to be produced in the United States through World War II. The
increasing demand for electricity also called for larger steam units in electric power plants. Here
too steam turbines replaced steam engines during the early part of the 20th century. Today a
single steam turbine-generator unit can produce more than 1 million kilowatts of electric power.
An example can be used to show the way in which steam produces work. If 1 pound of
steam is evaporated in a boiler at 450.F (232.C) to become all steam (saturated), then its
pressure will be 422.6 pounds per square inch (2,914 kilopascals) absolute and its volume will
be 1.099 cubic feet (0.031 cubic meter). If the steam is expanded ideally–that is, without
friction, cooling, or other losses–to atmospheric pressure, it will result in a mixture of water and
steam, called wet steam, at a temperature of 212.F (100.C) and allow 187,170 foot-pounds (254
kilojoules) of work to be extracted. However, its volume will have increased nearly twenty-fold.
On the other hand, if the same pound of steam can be expanded below atmospheric pressure to
2.0 pounds per square inch (13.8 kilopascals) absolute, then 269,760 foot-pounds (366
kilojoules) of energy can be extracted. The final temperature is 126.F (52.C) and the final
volume 129.8 cubic feet (3.65 cubic meters). Although more work is obtained in the latter
situation, gaining this extra work from each pound of steam requires the use of both a condenser
operating at below atmospheric pressures and a cooling source, which causes the steam to
condense back into a liquid form. (This water will then be pumped back into the boiler.) This
example illustrates an ideal case. In actual steam expansion, which involves cooling and other
losses, comparatively less work can be extracted and a somewhat different exhaust state results.
Steam engines with condensers are more efficient than steam engines without them. For
example, in locomotives steam exhausted to the outside air is wasted. Higher efficiency is also
possible if the steam expands to a lower temperature and pressure in the engine. The most
efficient performance–that is, the greatest output of work in relation to the heat supplied–is
secured by using a low condenser temperature and a high boiler pressure. The steam may be
further heated by passing it through a superheater on its way from the boiler to the engine. A
common superheater is a group of parallel pipes with the surfaces exposed to the hot gases in the
boiler furnace. Using superheaters, the steam may be heated beyond the temperature at which it
is produced by simply boiling water under pressure.
In a typical steam engine, steam flows in a double-acting cylinder. The flow can be
controlled by a single-sliding D valve. When the piston is in the left side of the cylinder,
high-pressure steam is admitted from the steam chest. At the same time, the expanded steam
from the right side of the cylinder escapes through the exhaust port. As the piston moves to the
right, the valve slides over both the exhaust ports and ports connecting the steam chest and the
cylinder, preventing more steam from entering the cylinder. The high-pressure steam within the
cylinder then expands. The steam expansion pushes the piston rod, which is usually connected to
a crank in order to produce rotary motion. When the valve is all the way to the left, steam in the
left-hand portion of the cylinder escapes as exhaust. At the same time, the right-hand portion of
the cylinder is filled with fresh high-pressure steam from the steam chest. This steam drives the
piston to the left. The position of the sliding D valve can be varied, depending on the position of
an eccentric crank on the flywheel.
Valve gearing plays a major role in a steam locomotive because a wide range of effort is
required of the engine. If the load on the engine is increased, the engine would tend to slow
down. The engine governor moves the location of the eccentric in order to increase the length of
time during which steam is admitted to the cylinder. As more steam is admitted, the engine
output increases. The efficiency of the engine decreases, however, because the steam can no
longer expand fully.
Although the D-slide valve is a simple mechanism, the pressure exerted by the
high-pressure steam on the back of the sliding valve causes significant friction losses and wear.
This can be avoided by using separate cylindrical spring-loaded spool valves enclosed in their
own chamber, as first proposed by George Corliss in 1849.
Arrangements more complicated than a simple eccentric are needed if a steam engine has
to run at different speeds and loads as well as forward and backward, as does a steam
locomotive. This leads to a complex arrangement of sliding valve levers, known as the valve
gear.
In a simple steam engine, expansion of the steam takes place in only one cylinder. In the
compound engine there are two or more cylinders of increasing size for greater expansion of the
steam and higher efficiency. Steam flows sequentially through these cylinders. The first and
smallest piston is operated by the initial high-pressure steam. Subsequent pistons are operated by
the lower-pressure steam exhausted from the previous cylinder. In each cylinder there is a partial
expansion and pressure drop. Since steam volume increases as the pressure is reduced, the
diameter of the low-pressure cylinders must be much larger if the engine stroke is to be the same
for all cylinders. In conventional compound engines the various cylinders are mounted side by
side and drive the same crankshaft.
The basic operation of steam turbines employs two concepts, which may be used either
separately or together. In an impulse turbine the steam is expanded through nozzles so that it
reaches a high velocity. The high-velocity, low-pressure jet of steam is then directed against the
blades of a spinning wheel, where the steam’s kinetic energy is extracted while performing work.
Only low-velocity, low-pressure steam leaves the turbine.
In a reaction turbine the steam expands through a series of stages, each of which has a
ring of curved stationary blades and a ring of curved rotating blades. In the rotating section the
steam expands partially while providing a reactive force in the tangential direction to turn the
turbine wheel. The stationary sections can allow for some expansion (and increase in kinetic
energy) but are used mainly to redirect the steam for entry into the next rotating set of blades. In
most modern large steam turbines, the high-pressure steam is first expanded through a series of
impulse stages–sets of nozzles that immediately lower the high initial pressure so that the
turbine casing does not have to withstand the high pressures produced in the boiler. This is then
followed by many subsequent impulse or reaction stages (20 or more), in each of which the
steam continues to expand.
The first reaction-type turbine was built by Hero of Alexandria in the 1st century AD. In
his aeolipile, steam was fed into a sphere that rotated as steam expanded through two
tangentially mounted nozzles. No useful work was produced by the aeolipile. Not until the 19th
century were attempts made to utilize steam turbines for practical purposes. In 1837 a rotating
steam chamber with exhaust nozzles was built to drive cotton gins and circular saws. A
single-stage impulse turbine was designed by the Swedish engineer Carl Gustaf de Laval in
1882. A later American design had multiple impulse wheels mounted on the same shaft with
nozzle sections located between each wheel. Subsequent advances in the design of steam
turbines and boilers allowed for higher pressures and temperatures. These advances led to the
huge and efficient modern machines, which are capable of converting more than 40 percent of
the energy available in the fuel into useful work.