Heat Exchanger Engineering Techniques
11 Manifolds and Headers
Manifolds, usually circular in cross-section, are devices intended to distribute flow as evenly as possible into two or more streams in parallel. Assume a selection has four exchangers in parallel. In this case the goal is to design a manifold such that one-fourth of the flow goes to each exchanger which would be good distribution. Poorly designed manifolds will not meet this goal. Heat exchanger headers (rectangular in cross-section or modified with rounded corners) act like manifolds as their purpose is to distribute flow equally to tubes in parallel. Poor distribution has been the cause of trouble in both manifold and header designs. These subjects are addressed in this chapter. In heat exchanger applications the term manifold refers to a piping arrangement designed to distribute flow to exchangers or sections, but not within headers. Manifolds are not part of the exchanger, nor are they normally furnished by the heat exchanger manufacturer. This said, simplifying their design and supports are part of the exchanger selection process. Headers and manifolds should be finalized together. When poorly designed, the exchanger may not and often does not deliver its full heating or cooling potential as will be illustrated by examples. Manifold and header designs are a governing conditions in the selection of air coolers.