A. INTRODUCTION

Heat exchangers transfer heat between two or more streams of fluid that flow through the apparatus. A major characteristic of heat exchanger design is the relative flow configuration, which is the set of geometric relationships between the streams. Section 88 describes the more common types of configuration.

It must be emphasized that the configurations described represent idealizations of what truly occurs; it is never possible, in practice, to make the flow patterns conform to the ideal.

B. COUNTERFLOW

In a counterflow heat exchanger, the two fluids flow parallel to each other, but in opposite directions. Figure 1 represents such a configuration schematically, by showing a single smaller-diameter tube placed coaxially within a tube of larger diameter. The two fluids flow respectively within the inner tube and through the annular space that separates the two tubes. In practice, a large number of tubes can be inserted within a single surrounding tube, of much larger diameter, known as the shell.