Environmental Engineering Reference
In-Depth Information
8.4 Mechanical Prime Movers: Engines and
Turbines
An enormous variety of combustion devices have been
designed to use released thermal energy directly in space
heating and industrial and agricultural processes or to
convert thermal energy to kinetic energy in mechanical
prime movers. The trend of these conversions has been
toward higher combustion temperatures and higher
power densities under more controllable conditions. For
example, small hand-stoked eighteenth-century units
burned lumps of coal on simple grates with densities of
about 100 kW/m
2
to heat unpressurized water, whereas
modern electricity-generating plant boilers consume
pulverized coal at rates up to 10 MW/m
2
to heat pres-
surized water to more than 600
C. The steam engine
was the first practical converter of coal into kinetic en-
ergy. Its slow introduction took up the entire eighteenth
century, and its subsequent rapid rise shaped the advance
of industrializing societies during the nineteenth century.
The steam engine has been the subject of many studies.
Dickinson (1939) and von Tunzelmann (1978) give per-
haps the best overviews; Jones (1973) offers a good col-
lection of illustrations. The evolution of the first practical
inanimate prime mover started with Denis Papin's 1690
experiments with a toy atmospheric. Papin's tiny device
was followed in 1698 by Thomas Savery's only partly
successful steam-operated pump, with a maximum rating
of about 750 W, and in 1712 by Thomas Newcomen's
engine, working at atmospheric pressure with a maxi-
mum effective output of 3.75 kW. Condensing steam on
the underside of the piston made Newcomen's engine
hugely inefficient (0.5%-0.7%), but John Smeaton's
improvements around 1770 doubled this performance.
James Watt's revolutionary contribution in his appro-
priately titled 1769 patent, ''A New Method of Lessen-
ing the Consumption of Steam and Fuel in Fire
Engines,'' was the introduction of a separate condenser
(fig. 8.8). An insulated steam jacket around the cylinder
and stuffing box, an air pump to maintain vacuum in the
condenser, sun-and-planet gearing, and later a double-
acting engine with steam moving the piston also on the
down stroke and a centrifugal governor to maintain con-
stant speeds with varying loads were other notable inno-
vations making Watt's engine a rapid commercial success.
By 1800, when the 25-year extension of the original pa-
tent expired, Watt and Matthew Boulton, his financing
partner, built about 500 engines (60% rotary, the rest
pumping), rated mostly at 8-16 kW (the largest ones
were just over 100 kW).
The steam engine's average rating of 20 kW was much
higher than the means of eighteenth-century watermills
(3.7 kW) or windmills (7.5 kW) but lower than the
power of many large waterwheels built to serve expand-
ing manufactures. But unlike windmills or waterwheels,
the engine allowed unprecedented freedom of location,
and its diffusion led to the emergence of new industrial
centers not only in or near major coalfields but also in
locations easily accessible by cheap water transport. An
intense period of innovation during the first half of the
nineteenth century, including Trevithick and Evans's
introduction of a high-pressure boiler in 1802 and Cor-
liss's invention of a valve mechanism in 1849, made the
steam engine more efficient and much more versatile. Its
portability, adaptability, dependability, and durability as-
sured its place as the most common prime mover of
nineteenth-century industrialization. There was little the
engines could not do; their original uses for pumping
in mines and for rotary power in textile mills were
soon augmented by a host of stationary and mobile
applications.
