Heat Exchanger
A Shell and Tube Heat Exchanger

Shell and tube heat exchangers consist of a series of tubes. One set of these tubes contains the fluid that must be either heated or cooled. The second fluid runs over the tubes that are being heated or cooled so that it can either provide the heat or absorb the heat required. A set of tubes is called the tube bundle and can be made up of several types of tubes : Plain, longitudinally finned etc. Shell and tube heat exchangers are typically used for high pressure applications with pressures greater than 30 bar and temperatures greater than 260°C. This is because the shell and tube heat exchangers are robust due to their shape.

There are several thermal design features that are to be taken into account when designing the tubes in the shell and tube heat exchangers. These include:

Tube Diameter :
Using a small tube diameter makes the heat exchanger both economical and compact. However it is more likely for the heat exchanger to foul up faster and the small size makes mechanical cleaning of the fouling difficult. To prevail over the fouling and cleaning problems, larger tube diameters can be used. Thus to determine the tube diameter, the available space, cost and the fouling nature of the fluids must be considered.

Tube Thickness :

• The thickness of the wall of the tubes is usually determined to ensure.
• Ability to easily stock spare parts cost sometimes the wall thickness is determined by the maximum pressure differential across the wall

Tube Length :
Heat exchangers are usually cheaper when they have a smaller shell diameter and a long tube length. Thus typically there is an aim to make the heat exchanger as long as possible. However there are many limitations for this including the space available at the site where it is going to be used and the need to ensure that ther are tubes available in lengths t hat are twice the required length so that the tubes can be withdrawn and replace. Also it has to be remembered that long thin tubes are difficult to take out and replace.

Tube Pitch :
When designing the tubes, it is practical to ensure that the tube pitch i.e the centre-centre distance of adjoining tubes is not less than 1.25 times the tubes outside diameter.