Modern Engineering Thermodynamics

13.6 Rankine Cycle with Superheat 467
Degree of superheat
T H
T L
T
Carnot cycle
1
A
Rankine
cycle with
superheat
3,4s 2s
p
4s
3
Carnot cycle
1
2s
T H = Constant
T L = Constant
s
(a) Temperature−entropy diagram for Carnot
and Rankine cycles with superheat
v
(b) Pressure−volume diagram for Carnot
and Rankine cycles with superheat
FIGURE 13.15
A comparison between the Carnot cycle and the isentropic Rankine cycle with superheat.
The degree of superheat is defined to be the difference between the actual superheated vapor temperature and the
saturation temperature at the pressure of the superheated vapor; that is,
Degree of superheat of superheated
= T − T
vapor at temperature T and pressure p
sat ðpÞ (13.10)
For example, in Figure 13.15a, the degree of superheat is T 1 – T A . The degree of superheat that can be used in
any particular heat engine design is limited only by the engine’s ability to resist high temperatures. This has led
to the industrial development and use of high-temperature alloys and ceramics for critical heat engine
components.
EXAMPLE 13.5
In the 1890s, the Lancashire Steam Motor Company (now called British
Leyland) manufactured the lawn mower shown in Figure 13.16, which
was powered by a Rankine cycle steam engine. If it had a superheated
boiler outlet state of 100. psia and 500.°F, and a condenser pressure of
1.00 psia, then find
a. The degree of superheat at the boiler outlet.
b. The equivalent Carnot cycle thermal efficiency of the lawn mower.
c. The isentropic Rankine cycle thermal efficiency of the lawn mower.
Solution
A system sketch is shown in Figure 13.17.
FIGURE 13.16
Example 13.5, steam-powered lawn mower.
W P
Q H
4 1
Boiler
Condenser
3
2
Q L
(a) Equipment schematic
1
T
W E
3,4s
2s
s
(b) T–s thermodynamic state diagram
FIGURE 13.17
Example 13.5, system sketch.
(Continued )