U Tube Heat Exchanger and Reboiler Heat Exchanger

• Contents :
• U Tube Heat Exchanger Construction and Working
• Design of U Tube Heat Exchanger
• Advantages and Disadvantages of U Tube Heat Exchanger
• Kettle type reboiler heat exchanger/ Reboiler heat exchanger Construction and Working.
• Design of Reboiler Heat Exchanger.
• Advantages and Disadvantages of Reboiler Heat Exchanger.

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• U tube heat exchanger :

A U tube heat exchanger (shown in Fig. 1 ) 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 pass partition and is used for entry and exit of the tube side fluid. At the opposite end of the shell, there is a cover which is integral with it. The shell is provided with two nozzles for entry and exit of the shell side fluid. The tube sheet is clamped between shell and channel flanges. The entire tube bundle can be removed from the shell from the channel end. The differential thermal expansion between 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 are less than those with a fixed tube sheet exchanger.

 In U tube heat exchanger, the tube side fluid enters through the channel, it flows first through one arm of the U - shaped tubes, it comes at the other end, travels down through the bend, flows through the other arm of the U-shaped tubes and ultimately leaves the exchanger through an outlet provided on the channel. During its passage through the exchanger, it exchanges heat with the shell side fluid. 

Fig. 1 : U tube heat exchanger 

The tube bundle can be removed for cleaning of tubes from the outside. The inside of the tube can be cleaned only by chemical means. 

 Commonly used for a reboiler on large fractionating columns (with an enlarged shell) where steam is condensed in the tubes. Also used for high pressure and temperature applications. 

• Kettle type Reboilers heat exchanger / Reboiler heat exchanger : 

It is either provided with an internal floating head 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 a column (i.e., for converting liquid into vapour). Fig. 5.12 shows a kettle type reboiler. It consists of a 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 shell and channel flange. In a channel, pass partition is incorporated so that inlet and outlet for the tube side fluid is provided on the same channel. At 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 liquid inlet and outlet connections at the bottom as shown in Fig. A vapour outlet is provided at the top. A weir is incorporated in the shell to maintain a pool of liquid in the shell so that the tube bundle remains submerged in the liquid. 

Fig. 2 : Kettle type reboiler heat exchanger with floating head arrangement 

Fig. 3 : Kettle type reboiler heat exchanger with U tube arrangement 

The heating medium, usually steam, flows through the tubes and the condensate is removed through a steam trap (not shown). The liquid to be vaporised is introduced in the enlarged shell through a 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 tube bundle. 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 product, through a liquid outlet provided at the bottom of the shell. 

• Also Read : Multiple Effect Evaporator