The use of a double-pipe heat exchanger is not limited to liquid-liquid heat exchange but may also be used for gas-liquid exchange and for gas-to-gas exchange.When the required heat-transfer surface is large, the recommended type of exchanger is the sell-and-tube variety. In this type of heater or cooler, large heat-transfer surface can be achieved economically and practically by placing tubes in a bundle; the ends of the tubes are mounted in a tube sheet. This is very commonly accomplished by expanding the end of the tube into a close-fitting hole in the tube sheet by a process called “rolling”. The resultant tube bundle is then enclosed by a cylindrical casing (the shell), through which the second fluid flows around and through the tube bundle. The fluid flowing through the tubes enters a header or channel where it is distributed through the tubes in parallel flow and leaves the unit through another header. Either the hot or the cold fluid may flow in the shell of the exchanger surrounding the tubes. Parallel flow through all tubes at a low velocity gives a low heat-transfer coefficient and low pressure drop. For higher rates of heat transfer, multi-pass operation may be used. Complexity in design sometimes results in expense in fabrication that must be balanced against improved performance and another disadvantage of the multi-pass exchanger is the added frictional loss due to higher linear velocities and the entrance and exit losses in the headers.If heat exchange is occurring between two fluids where one fluid has a very high resistance to heat transfer in comparison to the other, the higher-resistance. Experiment - Heat Transfer in a Plate Heat Exchanger.
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