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Heat exchangers are frequently included in the process flow sheets of industrial plants and have an important part to play. In cogeneration plants, the condensers attached to the exhaust from low pressure turbines recover not only the condensate, so reducing the cost of treating new makeup water, but also the heat that would otherwise be discharged into the environment and wasted. In many processes they are also used to minimize the heat lost to the atmosphere by recovering some of the heat contained in process discharge streams. Other heat exchangers are used to recover the heat contained in a process stream recycled from a late stage in the process and use it to preheat a stream entering an earlier stage in the process. In all cases, the increase in entropy engendered by the process is reduced and even minimized.
Heat exchangers can take many forms but, in all cases, the heat exchanger is presented with two fluid streams operating at different temperatures; the fluids are physically separated, the separation means also acting as a heat exchange surface. The separation commonly consists of bundles of tubes through which one of the fluids flows (See the shell and tube heat exchanger depicted in Figure 7.1). Sometimes finned tubes are used. The separation may also take the form of plates between which a fluid flows.