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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:
o There is enough room for corrosion
o That flow-induced vibration has resistance
o Axial strength
o 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 there are tubes
available in lengths that are twice the required
length (so that the tubes can be withdrawn and
replaced). Also, it has to be remembered that lone,
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
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