Page 241 - The Mechatronics Handbook

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chemical exergies. Standard chemical exergies are based on standard values of the environmental temper-
ature T 0 and pressure p 0 — for example, 298.15 K (25°C) and 1 atm, respectively. Standard environments
also include a set of reference substances with standard concentrations reflecting as closely as possible
the chemical makeup of the natural environment. Standard chemical exergy data is provided by Szargut
et al. (1988), Bejan et al. (1996), and Moran and Shapiro (2000).

Guidelines for Improving Thermodynamic Effectiveness
To improve thermodynamic effectiveness it is necessary to deal directly with inefficiencies related to
exergy destruction and exergy loss. The primary contributors to exergy destruction are chemical reaction,
heat transfer, mixing, and friction, including unrestrained expansions of gases and liquids. To deal with
them effectively, the principal sources of inefficiency not only should be understood qualitatively, but
also determined quantitatively, at least approximately. Design changes to improve effectiveness must be
done judiciously, however, for the cost associated with different sources of inefficiency can be different.
For example, the unit cost of the electrical or mechanical power required to provide for the exergy
destroyed owing to a pressure drop is generally higher than the unit cost of the fuel required for the
exergy destruction caused by combustion or heat transfer.
Chemical reaction is a significant source of thermodynamic inefficiency. Accordingly, it is generally
good practice to minimize the use of combustion. In many applications the use of combustion equipment
such as boilers is unavoidable, however. In these cases a significant reduction in the combustion irrevers-
ibility by conventional means simply cannot be expected, for the major part of the exergy destruction
introduced by combustion is an inevitable consequence of incorporating such equipment. Still, the exergy
destruction in practical combustion systems can be reduced by minimizing the use of excess air and by
preheating the reactants. In most cases only a small part of the exergy destruction in a combustion
chamber can be avoided by these means. Consequently, after considering such options for reducing the
exergy destruction related to combustion, efforts to improve thermodynamic performance should focus
on components of the overall system that are more amenable to betterment by cost-effective measures.
In other words, some exergy destructions and energy losses can be avoided, others cannot. Efforts should
be centered on those that can be avoided.
Nonidealities associated with heat transfer also typically contribute heavily to inefficiency. Accordingly,
unnecessary or cost-ineffective heat transfer must be avoided. Additional guidelines follow:

• The higher the temperature T at which a heat transfer occurs in cases where T > T 0 , where T 0
denotes the temperature of the environment, the more valuable the heat transfer and, consequently,
the greater the need to avoid heat transfer to the ambient, to cooling water, or to a refrigerated
stream. Heat transfer across T 0 should be avoided.
• The lower the temperature T at which a heat transfer occurs in cases where T < T 0 , the more
valuable the heat transfer and, consequently, the greater the need to avoid direct heat transfer with
the ambient or a heated stream.
• Since exergy destruction associated with heat transfer between streams varies inversely with the
temperature level, the lower the temperature level, the greater the need to minimize the stream-
to-stream temperature difference.
Although irreversibilities related to friction, unrestrained expansion, and mixing are often less signif-
icant than combustion and heat transfer, they should not be overlooked, and the following guidelines
apply:

• Relatively more attention should be paid to the design of the lower temperature stages of turbines
and compressors (the last stages of turbines and the first stages of compressors) than to the remaining
stages of these devices. For turbines, compressors, and motors, consider the most thermodynamically
efficient options.
• Minimize the use of throttling; check whether power recovery expanders are a cost-effective
alternative for pressure reduction.

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