Page 187 - Entrophy Analysis in Thermal Engineering Systems

182 Entropy Analysis in Thermal Engineering Systems

whereby heat is generated due to the frictional work. Thus, the entropy
generation due to pressure drop is directly related to an actual loss of useful
work, whereas that associated with heat transfer between the two fluids is
of no practical concern. Indeed, in a heat exchanger, the former should be
minimized while the latter is enhanced.

11.8.3 Application area of entropy analysis

The applicability of entropy-based analysis is limited. It may lead to mean-
ingful results in thermal power–generating systems and subject to certain
design constraints. For instance, as discussed in Chapter 8, in a power plant
driven by fuel combustion, the thermal efficiency is inversely proportional
to the specific entropy generation (SEG) defined as the ratio of the total entropy
generation rate per unit flowrate of the fuel burnt. For an engine operating
between two thermal reservoirs, the maximum efficiency design would be
identical to the minimum entropy generation design if either the heat input
or the work output is fixed. Furthermore, it must be noted that mere cal-
culation of the rate of entropy generation (or exergy destruction) is insuffi-
cient. Such calculations should enable designer to identify ways that would
boost the performance of a power-generating system, which would other-
wise be of no practical value.
Finally, the journey of this book comes to an end with an astonishing

note of Carnot as his final words in R eflexions sur la Puissance Motrice du
Feu, who about two centuries ago discouraged any attempt to mere achiev-
ing maximum theoretical power in heat engines [4,5].
We should not expect ever to utilize in practice all the motive power of combus-
tibles. The attempts made to attain this result would be far more hurtful than use-
ful if they caused other important considerations to be neglected. The economy of
the combustible is only one of the conditions to be fulfilled in heat-engines. In
many cases it is only secondary. It should often give precedence to safety, to
strength, to the durability of the engine, to the small space which it must occupy,
to small cost of installation, etc.

References
[1] R. Clausius, Ueber verschiedene f€ur die Anwendung bequeme Formen der
Hauptgleichungen der mechanischen W€armetheorie, Ann. Phys. (1865). Bd. cxxv, 353.
[2] M.J. Moran, H.N. Shapiro, D.D. Boettner, M.B. Bailey, Fundamentals of Engineering
Thermodynamics, ninth ed., Wiley, Hoboken, NJ, 2018.
[3] NIST Chemistry WebBook, SRD 69. National Institute of Standards and Technology,
https://webbook.nist.gov/chemistry/form-ser/.