INTRODUCTION
Heat exchangers in systems are used for:
1. recovering heat directly from one flowing medium to another or via a
storage system, or indirectly via a heat pump or heat transformer.
2. heating or cooling a .process stream to the required temperature for a
chemical reaction (this can also be direct or indirect).
3. enabling, as an intrinsic element, a power, refrigeration or heat pumping
process, that is interchanging heat between a hot source or stream with the
working fluid and with the low temperature heat sink (or source). Clearly
both power and refrigeration systems need both hot and cold streams: some
(heat transformers and absorption refrigeration systems) need 2 or more
sources or sinks.
In all applications of heat exchangers mechanical power is necessarily
expended to pump the working fluids through each exchanger by virtue of
pressure losses in its ducting or heat exchange passages. For liquids this power
is usually relatively small. In the ease of air or other gaseous (i.e. compressible
fluid) systems, however, the pumping power is often a significant design
variable, and its value- relative to the heat rate (or power) transferred- is a
commonly used (Kays and London (1984)) measure of the cost of primary
energy. The driving energy for pumps, fans and compressors is usually
electricity, which is of course produced with an efficiency loss than the Carnot
efficiency.
Many systems in the process industries have multiple streams exchanging
heat with each other and with "service" streams, that is, with streams of water,
steam or air specifically introduced to heat or cool. Some streams have theexpress function of absorbing power, or delivering power, as is the case with
compressors or turbines. In all cases a stream absorbs power as noted above.
These concepts point to the need for a rational way of analysing systems,
especially those involving both heat and power exchange, to enable operation at
minimum total energy consumption. Exergy analysis, a natural extension to
classical thermodynamic Second Law (entropy) analysis, is the unifying tool for
this purpose, and the basic concepts are developed in this chapter. The tool is
increasing being used in extended forms to include life- cycle analyses, that is, to
include the exergy cost of producing the (capital) equipment and its ultimate
disposal, but such development is beyond the scope of this book. The reader
wishing to take the subject further is recommended to study papers including
aspects of Thermoeconomics such as those by Witte (1988), Ranasinghe and
Reistad (1990), and books by Aheme, Szargut and Bejan et al (1996)
In this chapter the basic principles of exergy analysis are first outlined.
Their application to heat exchangers is then developed, firstly for zero pressure
drop and then for finite pressure drop for which an entropy generation minimum
criterion is derived, and its implications for design choices is discussed. Finally,
a brief discussion is given of the application of the principles to heat exchanger
networks.