8112Closed Heat Exchangers

As opposed to the open heat exchanger, the closed heat exchanger separates the stream containing the heating fluid from the stream containing the heated fluid, but allows the flow of heat across the separating boundaries. The reasons for separating the streams may be:

1. A pressure difference may exist between the two streams of fluid. The rigid boundaries of the heat exchanger are designed to withstand the pressure differences.

2. One stream could contaminate the other if allowed to mix. The impermeable, separating boundaries of the heat exchanger prevents mixing.

3. To permit the use of an intermediate fluid better suited than either of the principal exchange media for transporting waste heat through long distances. While the intermediate fluid is often steam, glycol and water mixtures and other substances can be used to take advantage of their special properties.

Closed heat exchangers fall into the general classification of industrial heat exchangers, however they have many pseudonyms related to their specific form or to their specific application. They can be called recuperators, regenerators, waste-heat boilers, condensers, tube-and-shell heat exchangers, plate-type heat exchangers, feedwater heaters, economizers, and so on. Whatever name is given all perform one basic function: the transfer of heat across rigid and impermeable boundaries. Sections 8.3 and 8.4 provide much more technical detail concerning the theory, application, and commercial availability of heat exchangers.

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