Thermal design frequently aims at
the most effective system from the cost viewpoint. Still, in the cost optimization
process, particularly of complex energy systems, it is often expedient to begin
by identifying a design that is nearly optimal thermodynamically; such a design
can then be used as a point of departure for cost optimization. Presented in
this section are guidelines for improving the use of fuels (natural gas, oil,
and coal) by reducing sources of thermodynamic inefficiency in thermal systems.
Further discussion is provided by Bejan et al. (1996).
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.
Since chemical reaction is a
significant source of thermodynamic inefficiency, 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 irreversibility 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 conventional 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 > T0 , where T0 denotes the
temperature of the environment (Section 2.5), 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
T0 should be
avoided.
• The lower the temperature T
at which a heat transfer occurs in cases where T < T0 , 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
streamto-stream temperature difference.
• Avoid the use of intermediate
heat transfer fluids when exchanging energy by heat transfer between two
streams
Although irreversibilities related
to friction, unrestrained expansion, and mixing are often secondary in
importance to those of 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.
• Avoid processes using
excessively large thermodynamic driving forces (differences in temperature, pressure,
and chemical composition). In particular, minimize the mixing of streams
differing significantly in temperature, pressure, or chemical composition.
• The greater the mass rate of
flow, the greater the need to use the exergy of the stream effectively.
• The lower the temperature level,
the greater the need to minimize friction. Flowsheeting or process
simulation software can assist efforts aimed at improving thermodynamic effectiveness
by allowing engineers to readily model the behavior of an overall system, or
system components, under specified conditions and do the required thermal
analysis, sizing, costing, and optimization. Many of the more widely used
flowsheeting programs: ASPEN PLUS, PROCESS, and
CHEMCAD are of the sequential-modular
type. SPEEDUP is a popular program of the equation-solver type. Since
process simulation is a rapidly evolving field, vendors should be contacted for
up-to-date information concerning the features of flowsheeting software,
including optimization capabilities (if any). As background for further
investigation of suitable software, see Biegler (1989) for a survey of the
capabilities of 15 software products.
