Shell and tube exchangers represent the history of thermal transfer, being the first ones that have been employed for cooling applications both in industry and conditioning.
Each typology has its own advantages and limits. The kind we use the most is the cleanable straight pipe exchanger: in this kind of equipment the two heads can be dismantled showing the straight tubes where the fluid flows, going from one head to the other. Being straight, they can be easily cleaned using a pipe cleaner, taking away the dirt and scaling. The diameter of the tubes depends on the kind of fluid to be treated, on the type of thermal transfer and process. These are usually welded exchangers, so that the tube bundle cannot be extracted. The clean fluid flows therefore inside the shell.
This kind of exchangers is commonly employed in cogeneration, for exhaust recovery on engines. The exhaust fumes flow inside the pipes, that are then cleaned. Another application is in the biogas sector, for biogas dehumidification. The biogas flows inside the tubes, while water glycol flows on the outside, within the shell.
Furthermore they are used for hot water production from vapour. Water flows inside the tubes and the vapour flows within the shell, heating the water that comes out at a higher temperature.
The limits of straight pipe cleanable shell and tube exchangers is related to their length, so that if long thermal transfers with long thermal gaps are required it is necessary to install multiple exchangers in series. The passage in series is possible, but in this case the construction of the heads becomes much more complicated.
The other classic typology is the U tube bundle exchangers. This kind of shell and tube exchanger has a bundle of U shaped pipes, welded upon a unique head. In this case, the section that can be cleaned is the shell, thus the external one, because the pipe bundle can be extracted. The pipes cannot be cleaned instead, because the curve cannot be reached using a pipe cleaner.
The dirty fluid usually flows inside the shell, even if when having bundles with a high amount of pipes, and very packed, cleaning operations can become very difficult to achieve.
This kind of exchanger allows to better leverage the thermal length, because here the thermal length of pipes doubles the overall length of the exchanger.
In terms of construction materials, shell and tube heat exchangers can employ mostly every kind of materials, from stainless steel to copper and iron. The selection depends on the kind of fluid to be treated, on the kind of process and the temperatures involved.
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