Modern Engineering Thermodynamics

464 CHAPTER 13: Vapor and Gas Power Cycles
efficiencies are in the range of 1%. Also note that the ratio of pump power to engine power in this example
is only
ð0:01672Þð20:0 − 14:7Þð144/778:16Þ Btu=lbm
j _W p / _W pm j =
ð1156:4 − 1133:9Þ − ð0:01672Þð20:0 − 14:7Þð144/778:16Þ Btu=lbm
= 0:00073 = 0:073%
and consequently the pump input power could be safely neglected in comparison to the engine output power
in the thermal efficiency calculation. Even so, for clarity and completeness, the pump power is included in all
subsequent thermal efficiency calculations carried out in the examples given in this chapter.
EXAMPLE 13.4
In 1876, George H. Corliss (1817–1888) built what was then the largest steam engine ever made (Figure 13.12), for the
United States Centennial Exposition in Philadelphia, Pennsylvania. It had two cylinders, each with a 40.0-inch bore and a
10.0 ft stroke. Steam entered the engine as a saturated vapor at 100. psia, producing 1400. hp at only 36.0 rpm. The engine’s
flywheel was 30.0 ft in diameter and weighed 56.0 tons. If the steam was condensed at 14.7 psia, determine the Rankine
cycle thermal efficiency of this engine assuming (a) isentropic prime mover and pump, (b) an engine isentropic efficiency of
55.0% and a pump isentropic efficiency of 65.0%, and (c) the steam mass flow rate required to produce 1400. hp.
FIGURE 13.12
Example 13.4, The Corliss engine.
Solution
a. The thermodynamic states at the four main points around the Rankine cycle for this system are (Figure 13.13)
Station 1—Engine inlet
p 1 = 100: psia
Station 2s—Engine exit
p 2s = 14:7 psia
x 1 = 1:00 s 2s = s 1
h 1 = h g ð100: psiaÞ = 1187:8 Btu/lbm
.
x 2s = s 2s – s f ð14:7 psiaÞ s fg ð14:7 psiaÞ
.
s 1 = s g ð100: psiaÞ = 1:6036 Btu/lbm . R = ½1:6036 − 0:3122Š 1:4447 = 0:8939
h 2s = h f ð14:7 psiaÞ + x 2s h fg ð14:7 psiaÞ
= 180:1 + 0:8939 ð970:4Þ
= 1047:5 Btu/lbm