Modern Engineering Thermodynamics

466 CHAPTER 13: Vapor and Gas Power Cycles
50
Reciprocating engines
Turbines
Thermal efficiency (%)
45
40
35
30
25
20
15
Atmospheric
(condensing) engines
Savery and
Newcomen
engines
The effect of
Watt’s
innovations
High−pressure
(expansion) engines
Isentropic
Rankine cycle
efficiency limit
Actual
thermal
efficiency
range
10
5
0
1700 1750 1800 1850 1900 1950 2000
Year
FIGURE 13.14
A chronology of steam engine thermal efficiency, 1700–2000.
The actual thermal efficiencies achieved by these early engines were naturally considerably less than that
predicted through an isentropic analysis. Figure 13.14 illustrates the growth of both the actual and the isentropic
efficiencies for the past three centuries. Since the early steam engines were very large and expensive, the
firm of Boulton and Watt devised a creative marketing scheme based on the superior thermal efficiency of
their engine. They let the purchaser pay for his engine by giving the company one third of the value of the
fuel saved with the new engine as compared with the fuel consumption of a standard Newcomen engine of
thesamesize.
13.6 RANKINE CYCLE WITH SUPERHEAT
Between 1850 and 1890, a variety of mechanical complexities were added to the reciprocating steam engine to
improve its thermal efficiency. For example, the cylinders were often staged in series, so that the steam was
first expanded in a high-pressure cylinder then exhausted to lower-pressure cylinder stages. Series staging of an
engine’s cylinders with the steam expanding only partially in each stage was then called compounding. Twostage
(duplex) expansion was introduced in 1811, three-stage (triplex) in 1871, and four-stage (quadruplex)
in 1875.
By 1880, it was recognized that initially dry saturated steam became wet when condensation occurred during the
expansion stroke of the piston (process 1 to 2 in Figure 13.8b). The water droplets thus formed inside the
cylinder tended to cool it slightly, and they promoted corrosion. This meant that the cylinder walls were being
alternately cooled and heated (as in the original Newcomen engine) slightly with each cycle of the engine, and
this reduced the engine’s thermal efficiency. However, if the steam entered the cylinder in a superheated state,
then the amount of moisture produced during the expansion stroke was greatly reduced or eliminated altogether.
Originally, the term surcharging was used to denote the use of superheated steam. This term has since
been replaced by the more direct term superheating.
Superheating the steam at the entrance to the cylinder alters the equivalent Carnot cycle by raising T H to the
superheating temperature and consequently increases the equivalent Carnot efficiency considerably. This, in
turn, makes the Rankine cycle appear less desirable by comparison, as shown in Figure 13.15. But all vapor
cycle heat engines operate on the Rankine cycle, and using the Carnot cycle for engineering comparison
purposes is purely academic. A more realistic comparison would be between the isentropic Rankine cycle and the
actual Rankine cycle.