Chapter 3 - The Heat Exchanger as Part of a System: Exergetic (Second Law) Analysis

Abstract

This chapter outlines the basic principles of energy analysis. The energy analysis applications to heat exchangers is 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 are discussed. Finally, the chapter presents a brief discussion of the application of the principles to heat exchanger networks. In all applications of heat exchangers, mechanical power is 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 systems, however, the pumping power is often a significant design variable, and its value relative to the heat rate transferred is a commonly used measure of the cost of primary energy. 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. These concepts point to the need for a rational way of analyzing systems, especially those involving both heat and power exchange, to enable operation at minimum total energy consumption.