Shell and tube heat exchangers are built of a shell in which several round tubes are mounted using tube sheets. Many variations of this basic type are available with differences lying mainly in constructional features and provisions for differential thermal expansion between shell and tubes.
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Types of Heat Exchanger
1. Fixed tube sheet Heat exchanger
2. Floating head heat exchanger
(a) Internal floating head and (b) Outside packed floating head.
3. U-tube type heat exchanger
4. Reboiler/Kettle type heat exchanger
(i) with an internal floating head or (ii) with a U-tube (hairpin).
The main components of all these exchangers are shell, shell cover, tubes, tube sheets, channel, channel cover, nozzles (inlet/outlet), pass partitions, baffles, tie rods and spacers.
1. Fixed tube-sheet exchanger
This is the simplest form of heat exchanger wherein the tube sheets are welded to a shell at both ends. The shell is equipped with two nozzles. The tube sheets usually extend beyond the shell and serve as flanges for the attachments of the channels on either side as shown in fig. In a fixed tube type construction, the shell and tube sheet materials must be weldable to each other.
The channels on either side are covered using channel covers and are provided with nozzle connections for the entry and exit of the tube side fluid. The shell side passes of more than two are rarely used but as such, there are no limitations on the tube side passes.
The other variation of the fixed tube sheet heat exchanger is shown in Fig. In which a tube side header may be welded to the tube sheet and channels on either side are closed with removable covers. To account for the differential expansion between shell and tubes occurring because of differences in the thermal properties of materials of shell and tubes, expansion joints such as expansion bellows are used on the shell.
Uses of Fixed shell and tube heat exchanger
This type of heat exchanger is used only when the shell side fluid is clean and non-fouling.
The disadvantage of fixed shell and tube heat exchangers
The problem with this type of heat exchanger is that the tube bundle can not be removed from the shell for mechanical cleaning and the usual inspection of the outside of the tubes.
2. Floating head heat exchanger
To make the removal of a tube bundle possible and to allow for a considerable expansion of the tubes, a floating head heat exchanger is used.
In this exchanger tubes are fixed in a floating tube sheet at one end and in a stationary tube sheet at the other end. The stationary tube sheet is clamped between a shell and a channel.
The floating tube sheet is clamped between a floating head and a clamp ring. The clamp ring which is split in half to permit dismantling is placed at the back of the tube sheet. They show details of a split ring assembly. The floating tube sheet is kept slightly smaller in diameter than the inside diameter of the shell to withdraw the entire tube bundle from the channel end. The channel is provided with inlet Andi outlet connections for the tube side fluid.
The shell is closed by a shell cover or bonnet on a floating head side. The shell cover at the floating head end is larger than the other end to enable the tubes to be placed as near as possible to the edge of the fixed tube sheet. The tube sheet along with the floating head is free to move and thus, this exchanger takes into account differential thermal expansion between shell and tube.
Uses of Floating head heat exchanger
It is widely used in the chemical industry and petroleum refineries. It is suitable for rigorous duties associated with high pressure and temperature and also with dirty fluids.
Advantages of floating head heat exchanger
i. The tube bundle of the exchanger is removable for inspection and mechanical cleaning of the tubes.
ii. It eliminates differential expression problems.
3. U-tube heat exchanger
A U-tube 1-2 heat exchanger shown in Fig. consists of U-shaped tubes, both the ends of which are fixed to a single stationary tube sheet. At one end of a shell, a channel is provided with a pass partition and is used for the entry and exit of the tube side fluid. At the opposite end of the shell, there is a cover which is integral to it. The shell is provided with two nozzles for entry and exit of the shell side fluid. The tube sheet is clamped between the shell flange and the channel flange.
The entire tube bundle can be removed from the shell from the channel end. The differential thermal expansion between the shell and tubes is absorbed by U-bends. Each tube can expand or contract freely without affecting the other tubes. As there is a limitation of bending tubes of a very short radius, the number of tube holes in the tube sheet of a given diameter with this exchanger is less than that with a fixed tube sheet exchanger.
The tube bundle can be removed to clean the tubes from the outside. The inside of the tube can be cleaned only by chemical means.
Use of U-tube heat exchanger
Commonly used for the reboiler on large fractionating columns where steam is considered in the tubes. Also used for high-pressure and temperature applications.
4. Kettle Reboiler/Reboiler heat exchanger
It is either provided with an internal floating heat arrangement or a U-tube arrangement. To provide a vapour space above a tube bundle, a shell is made larger in diameter.
In distillation operation, a reboiler is used to meet the latent heat requirements at the bottom of the column. It Shows Fig. a kettle-type reboiler. It consists of an enlarged shell containing a relatively small tube bundle. At one end of the bundle, the tubes are expanded into a stationary tube sheet clamped between the shell and channel flange. In a channel, a pass partition is incorporated so that the inlet and outlet for the tube side fluid are provided on the same channel.
At the opposite end of the bundle, tubes are expanded into a freely riding floating tube sheet. The tubes are free to expand. The shell is provided with a liquid inlet and outlet at the bottom as shown in Fig. A vapour outlet is provided at the top. A weir is incorporated into the shell to maintain a pool of liquid in it.
The heating media, usually steam, flows through the tubes and condensate is removed through a steam trap. The liquid to be vaporised is introduced in the enlarged shell through the liquid inlet. The tube bundle is always submerged in a pool of boiling liquid and for this purpose, an over-flow weir is incorporated in the shell, which is set aside of the floating head.
Heat transfer to boiling liquid takes place from a submerged surface. The shell is of a large diameter mainly for vapour-liquid separation. The vapours are generated, disengaged and removed from the top, and unvaporised liquid spills over the weir and is withdrawn as the bottom products through the liquid outlet provided at the bottom of the shell.
> Plate Type Heat Exchanger Working
Take these Notes is, Orginal Sources: Unit Operations-II, KA Gavhane