Page 173 - Chemical process engineering design and economics

156 Chapter 4

shown in Figure 4.3, are the flat gasket, the outside-packed stuffing box, and the
outside-packed lantern ring. The latter two seals are sliding seals, which allow for
movement between the sealing surfaces, thus relieving thermal stresses.
The maintenance required is cleaning, because of fouling of the heat-transfer
surface, replacing seals, and replacing or plugging leaky tubes. Because most heat
exchangers are overdesigned, some tubes could be plugged rather than replaced.
Cleaning can either be done chemically or mechanically.
Scale formation is referred to as fouling and may be caused by the following
mechanisms [25]:

1. precipitation of a salt from solution - frequently calcium carbonate in water
2. chemical reaction - such as polymerization of a monomer or corrosion, which
are accelerated by a warm surface
3. growth of a microorganisms
4. depositing of suspended matter

The fixed-tube-sheet heat exchanger, shown in Figure 4.1, is the most popu-
lar design. This heat exchanger has straight tubes sealed in tube sheets, which are
welded to the shell. Because the shell side is inaccessible for cleaning, we must
use clean fluids - such as steam, refrigerants, gases, and organic heat-transfer flu-
ids [16]. Differential thermal expansion must be considered when selecting a heat
exchanger. Because the shell and tubes may be made of different materials to re-
duce the cost, differential expansion could be considerable. Without an expansion
joint in the shell, the temperature difference between the shell and tube fluids is
limited to 80°C (144 °F) [17]. With an expansion joint, as shown in Figure 4.1, a
higher temperature difference is possible, but then the shell pressure is limited to
only 8.0 bar (7.90 arm) [17].
In the U-tube heat exchanger, shown in Figure 4.1, the tubes are free to ex-
pand within the shell to prevent thermal stresses. Because the tubes are bent, only
one tube sheet is needed, minimizing the number of connections. This feature plus
the gasket-type seal make this heat-exchanger suitable for high pressure applica-
tions. Maintenance, however, is more difficult than for other shell-and-tube heat
exchangers because any leaky inner tubes cannot be replaced, and must be
plugged. Mechanical cleaning in the tubes is also difficult because of the U-bends,
but chemical cleaning is possible. Also, hydraulic tube cleaners can clean both the
straight and curve part of the tubes [6].
Another way of relieving thermal stresses is to use an outside-packed stuff-
ing box or an outside-packed lantern ring, shown in Figure 4.1, and also in detail
in Figure 4.3. For both designs, one tube sheet is free to slide along the packing.
For the outside-packed stuffing box, the shell-side pressure is limited to 42.4 bar
(41.8 arm) and the temperature to 320°C (608 °F) [16]. If the packing leaks, the
shell and tube-side fluids will not mix. To clean the shell side of both heat ex-
changers, requires removing both ends and then sliding the tube bundle out of the
shell. The tubes and shell can be cleaned mechanically and the seals easily replaced.

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