Many different exchanger configurations can be easily created by different combinations of front end, shell and rear end. Moreover, depending on how tube bundle is fixed to the front end or rear end cover, we have 3 broad types of shell & tube heat exchanger construction.
Fixed Tube sheet Exchanger
A fixed-tube sheet heat exchanger has straight tubes that are secured at both ends to tube sheets welded to the shell.
The main advantage of the fixed tube sheet exchanger is its low cost because of its simple construction. Fixed tube sheet being the least expensive shell & tube exchanger type, as long as we don't use any expansion joint. But for the same reason, fixed tube sheet exchanger does not help much for services having large temperature difference between shell & tube sides. Because in such cases, an expansion joint would be needed. Other advantages are that the tubes can be cleaned easily by removing the channel cover or bonnet. Moreover, absence of flanged joints helps to minimize the leakage of the shell side fluid. But there is a disadvantage that the outsides of the tubes cannot be cleaned mechanically, as the tube bundle is fixed to the shell. Fixed tube sheet exchangers require that a clean fluid must be used on the shell side.
When you have a dirty fluid on tube side, you may consider this type of exchanger.
U tube exchanger
As the name suggests, in this type the tube bundle is U shaped. There is only one tube sheet. All the tubes start from upper half of this tube sheet, make a U turn in the shell and come back to the lower half of the same tube sheet. As shown in the following diagram.
The advantage of using a U tube bundle is that the tubes can freely expand as they are U turn end of the bundle is freely floating in the shell side. Hence a U tube exchanger is the preferred option where there is high temperature difference between shell & tube side fluids and tube expansion is expected.
But as the same time, this U shape of the tubes makes it difficult to mechanically clean them. Only chemical cleaning would be possible. Hence U tube exchangers are normally not preferred where we need to use a dirty or fouling service on the tube side.
These exchangers are also cost effective, as expansion joints are not needed and tube bundle is free to expand.
Floating head exchanger
In this type of shell & tube exchanger, one end of the tubes is kept fixed in a tube sheet attached to the shell side. While the other end is free to expand or 'floating' in the shell side.
Thanks to this design, this type of shell & tube heat exchangers can withstand fluids to high temperature difference, as the tubes are free to expand. Also, the floating head cover can be easily removed to mechanically clean insides of the tubes. Therefore, even dirty and fouling services can also be used on the tube side.
That makes this shell & tube heat exchanger type most versatile in terms of its applicability to different scenarios.
But the design is quite complex, making it also the most expensive type of shell & tube exchangers.