Modern Engineering Thermodynamics

454 CHAPTER 13: Vapor and Gas Power Cycles
WHO WAS JAMES WATT? Continued
The Boulton and Watt company was very successful
and manufactured steam engines for many years, including
the engine used in 1807 by Robert Fulton (1765–
1815) on the first American steamboat, the Clermont
(Figure 13.5).
Over the years Watt made many important technological
advances including automatic controls (the centrifugal
governor), safety devices (the pressure gauge), and efficiency
improvements (the double-acting cylinder).
FIGURE 13.5
An illustration of Robert Fulton’s steamboat Clermont.
The term horsepower was introduced by Savery as a measure
of how many horses driving a mechanical pump were
replaced by his fire engine. However, as a unit of power
measurement, it lacked a precise definition until Watt carried
out experiments in about 1780 to determine how
much power an average horse could deliver on a continuous
basis. He then multiplied this value by a factor of 2 to
ensure that, when he sold engines rated at a given horsepower,
the purchasers would have no complaints as to their performance. Using this technique, he finally arrived at
the figure of 33,000 ft·lbf/min as his horsepower definition. In addition to his conservative two-horse horsepower
definition, some other equivalences that Watt felt were valid are 1 horsepower = 2 average horses = 3 powerful oxen = 12 men
working cranks = 396 gallons of water falling 10 ft in 1 min. Much later, the electrical unit of power was named after
him, 1 watt = 1volt· ampere (and 1 hp = 746 W).
By 1800, Watt’s improvements had increased the thermal efficiency of a full-size Newcomen steam engine by about a factor
of 4. But, even then, the overall thermal efficiency was only around 4 or 5%. This was to be the upper limit of atmospheric
engine thermal efficiency, because they were soon to be replaced by a new technological breakthrough: the high-pressure,
expansion steam engine. 5
4 This occurred because small-scale models magnify the inefficiencies that arise from heat loss from the engine’s surface to the atmosphere. Since the
surface area to volume ratio of a given geometric shape always increases as the physical dimensions of the shape decrease, small heat engine models are
inherently much less efficient than their full-scale counterparts.
5 It has been estimated that the upper limit of the thermal efficiency of the reciprocating atmospheric steam engine using the technology available in 1915
was less than 15%.
BOILER EXPLOSIONS AND EXPANSION STEAM ENGINES
Before the early 19th century, high-pressure steam (anything over about 10 psig) was considered extremely dangerous.
Explosions of the primitive boilers of Savery’s time caused much damage and loss of life and traumatized steam engine
manufacturers. But new boiler materials and manufacturing and testing techniques in the early 1800s allowed operational
steam pressures many times the single atmosphere of pressure available to Watt’s engines. Engines of 100 psig were common
by 1840, and 200 psig was in use by 1880. High-pressure engine technology brought another quantum leap in thermal
efficiency, another factor of 3, from Watt’s 4 to 5% in 1800 to 12 to 15% by 1850.
High-pressure engines used the pressure of the steam to push the piston by expanding against it, rather than having atmospheric
pressure push the piston into a vacuum as the atmospheric engines had done. Thus, they were known as expansion
engines. By the 1820s, high-pressure engines were sufficiently efficient that Watt’s condenser unit was no longer considered
to be essential, and consequently the steam was often exhausted from the cylinder directly into the atmosphere. The condenser
always increased the engine’s thermal efficiency somewhat, but if the piston-cylinder was not cooled during each
cycle (as it was in the Newcomen atmospheric engines but not in the newer expansion engines), this effect was minimal.
The elimination of the condenser and its attendant pump simplified the engine’s construction and further reduced its cost
and size, with only a small loss in thermal efficiency. At this point, the steam engine was finally small enough to become
truly portable and its application to locomotion on land (railroads) and water (steamboats) produced new transportation
technologies that changed the face of the world.