A. INTRODUCTION

The purpose of this section is to draw attention to the fact that the fluid streams in a heat exchanger are brought into contact (direct or indirect) in a wide variety of ways. This purpose can best be met by describing fluid-interface types briefly, and indicating where more complete descriptions can be found.

B. PLAIN TUBES

The most common arrangement is for one fluid to flow within straight or curved tubes of circular cross section, while a second fluid washes the outside of these tubes in a longitudinal, perpendicular, or oblique direction.

The interface is thus represented by a tube wall, and the fluids physical contact is indirect. The tubes may, of course, be noncircular in cross section; often they are bent into hairpin shapes and sometimes they take the form of helical coils.