Heat Recovery Heat Exchangers

There are many types of heat exchangers, although their common function is the transfer of heat from one medium to another. This usually requires both fluids to flow past a separating membrane that provides conductive transfer from the high-temperature fluid to the low-temperature fluid. The rate of heat transfer depends on the logarithmic mean temperature difference, ATLmtd, between the fluids.

Flow arrangements may be characterized as parallel flow, counterflow, crossflow, and mixed flow. Counterflow is the most efficient heat exchanger arrangement, producing the greatest temperature difference and requiring the least heating surface. Heat exchangers can also be classified as being of either direct-contact or indirect-contact design.

Direct-contact applications involve some type of mixing of fluid streams: a deaerator might be considered a type of heat exchanger in that steam and condensate are mixed to produce a heated feedwater stream. Another example is the heat wheel, in which a rotating medium is heated by direct contact with one gas or air stream and then moved to contact and heat a second air stream.

Indirect-contact heat exchangers include separate circuits for each fluid stream, allowing fluids to be at different pressures and preventing mixing or contamination. Important types of indirect-contact heat exchangers include the following:

• Shell-and-tube heat exchangers include a cylinder, or shell, containing a tube bundle. Generally, the higher-pressure fluid flows through the tubes with the lower-pressure fluid in the shell. When the heating fluid is steam, flow will usually be through the shell. There are a wide variety of shell-and-tube configurations, sizes, and materials of varying cost and effectiveness.

• Concentric tube (double pipe or multi-tube) heat exchangers can be used for handling high-pressure fluids. Designs include a single tube or bundles of tubes of a single length, a spiral coil, or a tube bundle with hairpin bends. The flow arrangement can be either parallel or counterflow. Concentric tube heat exchangers typically offer the benefit of easy disassembly.

• Plate-and-frame liquid/liquid heat exchangers include contoured plates with gasketed edges that are clamped tightly together in a frame. Hot and cold fluids are directed through the spaces between the plates, allowing heat transfer at high effectiveness.

• Finned-tube HRSGs are commonly used in large power generation systems. The design, which resembles a shell-and-tube heat exchanger, is used to generate steam from high-temperature exhaust gas.

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