A. Selection of a basic type of heat exchanger

The most important decision underlying design of a piece of heat transfer equipment is the selection of the basic type of equipment to be specified and designed for a given application. It is incumbent upon the designer, at a very early stage in the design process, to survey the range of basic equipment types available and to select the one most applicable to his or her particular process. If a clear-cut decision cannot be made, it will probably prove economically desirable to proceed with at least first-stage design on each type of equipment that may reasonably serve.

A consideration that often enters into the selection of a basic type is the availability of comprehensive and accurate design methods for that equipment. Thus, shell-and-tube exchangers, for which a generally very good design procedure is available, are often selected for a service in preference to another type that may be intrinsically preferable in the application but which lacks a comparable design method in which the designer may place confidence. There is justification for this philosophy, but it can be bought at too high a price. Most heat exchanger types have good design methods available for most applicable services, though the best methods are often proprietary to the manufacturers or to members of cooperative research organizations.

B. Double-pipe heat exchangers

A typical double-pipe heat exchanger is shown in Figure 1. Essentially, it consists of one pipe placed concentrically inside another one of larger diameter, with appropriate end fittings on each pipe to guide the fluids from one section to the next. The inner pipe may have longitudinal fins welded, brazed, or soldered to it either internally or externally to increase the heat transfer area for the fluid with the lower heat transfer coefficient. The double-pipe sections can be connected in various series or parallel arrangements for either fluid to meet pressure drop limitations and MTD requirements. The major use of double-pipe exchangers is for sensible heating or cooling of the process fluid where small heat transfer areas (typically up to 50 m²) are required. They may also be used for small amounts of boiling or condensation on the process fluid side.