Online Encyclopedia
Steam engine
A steam engine is a heat engine that makes use of the potential energy that exists as pressure in steam, converting it to mechanical work. Steam engines were used in pumps, locomotive trains and steam ships, and were essential to the Industrial Revolution. They are still used for electrical power generation.
A steam engine needs a boiler to boil water to produce steam under pressure. Any heat source can be used, but the most common is a wood or coal fire. The steam is allowed to expand by pushing against a piston or turbine, whose motion is used to do work.
| Contents |
Invention
The first steam device, the aeolipile, was invented by Heron of Alexandria, a Greek, in the 1st century AD, but used only as a toy.
Denis Papin, a French physicist, built a working model of a steam engine after observing steam escaping from his pressure cooker in about 1679. Early industrial steam engines were designed by Thomas Savery (1698), Thomas Newcomen (1712), and James Watt (1769), each adding new refinements.
Early engines worked by the vacuum of condensing steam, whereas later types (such as steam locomotives) used the power of expanding steam.
Use and Development
The first industrial applications of the vacuum engines were in the pumping of water from deep mineshafts. The Newcomen engine operated by admitting steam to the operating chamber, closing the valve, and then admitting a spray of cold water. The water vapor condenses to a much smaller volumn of water, creating a vacuum in the chamber. Atmospheric pressure, operating on the opposite side of a piston, pushes the piston to the top of the chamber. In mineshaft pumps, the piston was connected to an operating rod that descended the shaft to a pump chamber. The oscillations of the operating rod are transfered to a pump piston that moves the water, through check valves, to the top of the shaft. The first significant improvement was creation of a separate condensing chamber with a valve between the operating chamber and the condensing chamber. This improvement, by Watt, significantly increased the efficiency of the engine. The next improvement was the replacement of manually operated valves with valves operated by the engine itself. Vacuum (condensing) engines are severely limited in their efficiency but are relatively safe since the steam is at very low pressure and structural failure of the engine will be by inward collapse rather than an outward explosion.
Further improvements in efficiency came with the use of pressurized steam, but with this came much danger and many disasters due to exploding boilers and machinery. The most important refinement at this point was the safety valve, which releases excess pressure. Reliable and safe operation came only with a great deal of experience and codification of construction, operating, and maintainance procedures.
Technology
Boilers are of two types:
- Fire tube construction is typical of early maritime installations for boats and ships and the boilers of steam locomotives. In a fire tube boiler, the hot gases from the combustion chamber are passed through tubes connecting perforated end plates. The gases then enter a "smoke chest" and pass on to a smokestack. The boiler may be vertical or horizontal. For an example of a vertical boiler of this type observe the boiler in the small riverboat used in the movie The African Queen. This type is also used in some boilers that provide steam for steam heating of a building. Locomotives and ships generally use a horizontal orientation and ships will usually require a tall smokestack to provide draft, not having a fan to provide a forced draft. In a steam locomotive the draft may be augmented by the steam exhaust.
- In a water tube boiler the water is heated in multiple tubes exposed to the hot gases. The tubes are joined to a steam collector chamber at the top. A significant advantage of this type is that there is less chance of catastrophic failure, as there is not a great amount of water in the boiler, nor are there large mechanical elements subject to failure. There may be additional tubes above the collector in the upper portion of the hot gas exhaust - this device, called a superheater, provides additional temperature (and hence pressure) and increases the thermal efficiency of the entire mechanism.
High pressure steam engines are of various types but most are either reciprocating piston or turbine devices.
Most reciprocating engines use double acting pistons, with pressurized steam admitted alternately to each side while the other side is exhausted to the atmosphere or to a condenser. Power is removed by a sliding rod, sealed against the escape of steam. This rod in turn drives (via a sliding crosshead bearing) a connecting rod connected to a crank to convert the reciprocating motion to rotary motion. An additional crank or eccentric is used to drive the valve gear, usually through a reversing mechanism to allow reversal of the rotary motion.
Steam turbines for high power applications will use a number of rotating disks containing propeller-like blades at their outer edge. These "rotor" disks alternate with "stator" blade rings affixed to the turbine case that serve to redirect the steam flow for the next stage. Owing to the high speed of operation such turbines are usually connected to a reduction gear to drive another mechanism such as a ship's propeller. Steam turbines are more durable, smoother operating, and require far less maintenance than reciprocating engines.
Steam powered vehicles
Nicolas-Joseph Cugnot demonstrated the first functional self-propelled steam vehicle, his "steam wagon", in 1769. Arguably, this was the first automobile. While not generally successful as a transportation device, the self-propelled steam tractor proved very useful as a self mobile power source to drive other farm machinery such as grain threshers or hay balers.
Steam engine powered automobiles continued to compete with other motive systems into the early decades of the 20th century. However steam engines are less favored for automobiles, which are generally powered by internal combustion engines, because steam requires at least thirty seconds (in a flash boiler) or so to develop pressure.
On February 21, 1804 at the Pen-y-Darren ironworks in Wales, the first self-propelled railway steam engine or steam locomotive built by Richard Trevithick was demonstrated.
Advantages
The strength of the steam engine for modern purposes is in its ability to convert heat from almost any source into mechanical work. Unlike the internal combustion engine, the steam engine is not particular about the source of heat. Most notably, without the use of a steam engine nuclear energy could not be harnessed for useful work, as a nuclear reactor does not directly generate either mechanical work or electrical energy - the reactor itself simply heats water. It is the steam engine which converts the heat energy into useful work. Steam may also be produced without combustion of fuel, through solar concentrators. A demonstration power plant has been build using a central heat collecting tower and a large number of solar tracking mirrors, (called heliostats).
Efficiency
A steam engine exhausting to atmosphere will have an efficiency (including the boiler) of 5% but with the addition of a condenser the efficiency is greatly improved to 25% or better. A power station with exhaust reheat, etc. will achieve 30% efficiency.
One source of inefficiency is that the condenser causes losses by being somewhat hotter than the outside world. Thus any closed-cycle engine will always be somewhat less efficient than any open-cycle engine, because of condenser losses.
The operation of the engine portion alone is not dependant upon steam; any pressurised gas may be used. Compressed air is sometimes used to test or demonstrate small model "steam" engines.
See also
- Newcomen steam engine
- Watt steam engine
- Stationary steam engine
- Steam Locomotive for details of steam powered railway 'engines'
- Crosshead bearing
External Links
The World's Smallest Steam Engine